Revert mention of drag'n'drop epics

What does this MR do?

Removes mention of the ability to reorder and epic's child epics, and child issues.

This was documented in !17297 (merged). The associated feature was due in %12.4 but has been postponed to %12.5.

I have chosen not to revert the entire MR because several cosmetic changes were also applied in that MR. I would prefer that those changes remain, so the scope of this MR is quite small.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• [-] Code review guidelines
• Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content