Revert of the v9.0 backport of SERVER-125965 on the v9.0-staging branch.

Reverts v9.0 commit a0ad6c116cbef6833ee7d14d8bd046c7ef099441 ("SERVER-125965: Wait for stable timestamp to advance in initial sync (#54176)").

This mirrors the revert already landed on master as PR #61087 (824731d3577e). SERVER-125965 is a sub-task of SERVER-113184, whose initial sync changes have been backed out of master; the v9.0 branch is being brought back in line.

Removes the wait-for-stable-timestamp logic from initial_syncer.cpp/.h and initial_sync_state.h, the associated unit tests, the test changes in initial_sync_drop_collection.js, initial_sync_during_stepdown.js and initial_sync_rename_collection.js, and the corresponding etc/backports_required_for_multiversion_tests.yml entries.

Note: jstests/replsets/initial_sync_waits_for_stable_timestamp.js, added by this backport, is deleted in the SERVER-127512 revert commit on the branch rather than here.