Feature flag: removal of get_tag_messages_go

What does this MR do?

Gitaly was migrated to new implementation, no need in flag anymore.

Please see related issue: gitaly#2208 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

Nothing to be broken. The flag is not used.