Remove On-call Edit feature flag [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Reverts !55243 (merged), thus removing the :oncall_schedules_mvc_edit_rotations feature flag.

This feature flag was not used, no other code/spec changes are required.

issue: #283914 (closed) discussion: !56263 (comment 527705000)

Screenshots (strongly suggested)

No visible change.

Does this MR meet the acceptance criteria?

Conformity

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Security

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