Drop un-used db/ci_migrate symlink

What does this MR do?

Drop un-used db/ci_migrate symlink

1. The db/ci_migrate symlink is not currently used in production
2. It was supposed to be used in development for the decomposition efforts. But it seems we don't need it for our current purposes, as Rails will default migrations_paths to db_migrate anyway. See
   • https://github.com/rails/rails/blob/6-1-stable/activerecord/lib/active_record/railties/databases.rake#L26
   • https://github.com/rails/rails/blob/6-1-stable/activerecord/lib/active_record/tasks/database_tasks.rb#L86
   • https://github.com/rails/rails/blob/6-1-stable/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb#L130-L131

Resolves #339079 (closed)

Screenshots or Screencasts (strongly suggested)

Impact on development.

With multiple databases, I was able to run db:migrate, db:migrate:redo:cianddb:migrate:redo:main` for a new migration fine:

tkgl:gitlab tkuah$ be rails db:migrate:redo:ci
== 20210823002133 AddSomeRandomTable: reverting ===============================
-- drop_table(:randoms)
   -> 0.0049s
== 20210823002133 AddSomeRandomTable: reverted (0.0079s) ======================

== 20210823002133 AddSomeRandomTable: migrating ===============================
-- create_table(:randoms)
   -> 0.0086s
== 20210823002133 AddSomeRandomTable: migrated (0.0087s) ======================


tkgl:gitlab tkuah$ be rails db:migrate:redo:main
== 20210823002133 AddSomeRandomTable: reverting ===============================
-- drop_table(:randoms)
   -> 0.0040s
== 20210823002133 AddSomeRandomTable: reverted (0.0065s) ======================

== 20210823002133 AddSomeRandomTable: migrating ===============================
-- create_table(:randoms)
   -> 0.0064s
== 20210823002133 AddSomeRandomTable: migrated (0.0065s) ======================

gitlabhq_development=# \c gitlabhq_development_ci
You are now connected to database "gitlabhq_development_ci" as user "tkuah".
gitlabhq_development_ci=# \d randoms
                                       Table "public.randoms"
   Column   |           Type           | Collation | Nullable |               Default               
------------+--------------------------+-----------+----------+-------------------------------------
 id         | bigint                   |           | not null | nextval('randoms_id_seq'::regclass)
 created_at | timestamp with time zone |           | not null | 
 updated_at | timestamp with time zone |           | not null | 
Indexes:
    "randoms_pkey" PRIMARY KEY, btree (id)

Example below:

1. Enable the invite modal

   Feature.enable(:invite_members_group_modal)

2. In rails console enable the experiment fully

   Feature.enable(:member_areas_of_focus)

3. Visit any group or project member pages such as http://127.0.0.1:3000/groups/flightjs/-/group_members
4. Click invite members button.

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