Remove the git_push_execute_all_project_hooks feature flag

What does this MR do?

This reverse feature flag has been operating in production for at least 4 months. It should be safe to remove, per discussion in #34241 (comment 305692993) .

Since there's no behaviour change with the removal of this flag, there is no need for a changelog entry.

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

Closes #34241 (closed)