Remove export_lfs_objects_projects feature flag

What does this MR do?

This MR removes the export_lfs_objects_projects feature flag.

The flag has been enabled by default for 1 year since d18ee3fa, so will be safe to remove.

• Issue: #11090 (closed).

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

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done