Remove the trace chunks migration flag
The migration capability implemented in #1095 (closed) needs to be either:
- Documented, for others to follow later, or
- Removed (the feature flag, basically).
This must be done before we can consider the Epic completed, but can be done as a cleanup (non-blocking on the actual migration)
The migration code will be used by GitLab.com to perform this migration. Once the migration is completed, we (GitLab.com) won't need this code again. The only reason to leave it in place in the code base is if other large GitLab installations will need to perform the same migration.
The migration might be needed by installations where their CI trace chunks make up a large enough portion of redis-persistent traffic - for example, trace chunks are between 50% and 60% of our traffic.
We're going to go ahead with removing the migration code: #1131 (comment 605067774)