Delete index_unique_project_authorizations_on_project_id_user_id

What does this MR do and why?

!119317 (merged) introduced the index_project_authorizations_on_project_user_access_level index which supersedes the existing index_unique_project_authorizations_on_project_id_user_id index. This MR removes the superseded index.

Table "public.project_authorizations"
    Column    |  Type   | Collation | Nullable | Default | Storage | Stats target | Description 
--------------+---------+-----------+----------+---------+---------+--------------+-------------
 user_id      | integer |           | not null |         | plain   |              | 
 project_id   | integer |           | not null |         | plain   |              | 
 access_level | integer |           | not null |         | plain   |              | 
Indexes:
    "project_authorizations_pkey" PRIMARY KEY, btree (user_id, project_id, access_level)
    "index_project_authorizations_on_project_user_access_level" UNIQUE, btree (project_id, user_id, access_level)
    "index_unique_project_authorizations_on_project_id_user_id" UNIQUE, btree (project_id, user_id)
Foreign-key constraints:
    "fk_rails_0f84bb11f3" FOREIGN KEY (project_id) REFERENCES projects(id) ON DELETE CASCADE
    "fk_rails_11e7aa3ed9" FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE
Access method: heap
Options: autovacuum_enabled=true

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Related to #395545 (closed)