Remove GitlabIssueTrackerService class

Description

This merge request was originally introduced in !34852 (merged) but we found a problem related to the single-table inheritance mechanism #225169 (closed) and reverted the changes !35510 (merged).

We have been discussing how to proceed with the changes !35221 (comment 379184235) and this is the final plan #218526 (comment 380963641):

1. Remove the records in a post-migration (!35221 (merged)). %13.2
2. Remove the GitLabIssueTrackerService class (This merge request !36063 (merged)). %13.3
3. Run the migration again to remove any existing records in a post-migration (!37931 (merged)). %13.3

Related to #218526 (closed)