Cleanup PK conversion for events table

What does this MR do and why?

For GitLab.com, remove events.target_id_convert_to_bigint integer columns and the related trigger.

This step from the process to convert column to bigint is described here - https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html#remove-the-trigger-and-old-integer-columns-release-n--2.

The column swaps for this table were done with Swap events.target_id to bigint (!126786 - merged).

UPDATE Jul. 25th 19:32 UTC

• target_id was converted to a bigint
• index_events_on_target_type_and_target_id_bigint_fingerprint was created and renamed to index_events_on_target_type_and_target_id_and_fingerprint

Table "public.events"
           Column            |           Type           | Collation | Nullable |              Default               
-----------------------------+--------------------------+-----------+----------+------------------------------------
 project_id                  | integer                  |           |          | 
 author_id                   | integer                  |           | not null | 
 target_id_convert_to_bigint | integer                  |           |          | 
 created_at                  | timestamp with time zone |           | not null | 
 updated_at                  | timestamp with time zone |           | not null | 
 action                      | smallint                 |           | not null | 
 target_type                 | character varying        |           |          | 
 group_id                    | bigint                   |           |          | 
 fingerprint                 | bytea                    |           |          | 
 id                          | bigint                   |           | not null | nextval('events_id_seq'::regclass)
 target_id                   | bigint                   |           |          | 
Indexes:
    "events_pkey" PRIMARY KEY, btree (id)
    "index_events_on_target_type_and_target_id_and_fingerprint" UNIQUE, btree (target_type, target_id, fingerprint)
    "analytics_index_events_on_created_at_and_author_id" btree (created_at, author_id)
    "index_events_author_id_project_id_action_target_type_created_at" btree (author_id, project_id, action, target_type, created_at)
    "index_events_for_followed_users" btree (author_id, target_type, action, id)
    "index_events_for_group_activity" btree (group_id, target_type, action, id) WHERE group_id IS NOT NULL
    "index_events_for_project_activity" btree (project_id, target_type, action, id)
    "index_events_on_author_id_and_created_at" btree (author_id, created_at)
    "index_events_on_author_id_and_created_at_merge_requests" btree (author_id, created_at) WHERE target_type::text = 'MergeRequest'::text
    "index_events_on_author_id_and_id" btree (author_id, id)
    "index_events_on_created_at_and_id" btree (created_at, id) WHERE created_at > '2021-08-27 00:00:00+00'::timestamp with time zone
    "index_events_on_group_id_and_id" btree (group_id, id) WHERE group_id IS NOT NULL
    "index_events_on_group_id_partial" btree (group_id) WHERE group_id IS NOT NULL
    "index_events_on_project_id_and_created_at" btree (project_id, created_at)
    "index_events_on_project_id_and_id" btree (project_id, id)
    "index_on_events_to_improve_contribution_analytics_performance" btree (project_id, target_type, action, created_at, author_id, id)
Check constraints:
    "check_97e06e05ad" CHECK (octet_length(fingerprint) <= 128)
Foreign-key constraints:
    "fk_61fbf6ca48" FOREIGN KEY (group_id) REFERENCES namespaces(id) ON DELETE CASCADE
    "fk_edfd187b6f" FOREIGN KEY (author_id) REFERENCES users(id) ON DELETE CASCADE
    "fk_rails_0434b48643" FOREIGN KEY (project_id) REFERENCES projects(id) ON DELETE CASCADE
Referenced by:
    TABLE "push_event_payloads" CONSTRAINT "fk_36c74129da" FOREIGN KEY (event_id) REFERENCES events(id) ON DELETE CASCADE
Triggers:
    trigger_cd1aeb22b34a BEFORE INSERT OR UPDATE ON events FOR EACH ROW EXECUTE FUNCTION trigger_cd1aeb22b34a()

Related to #414777 (closed).

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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