Revert of the v9.0 backport of SERVER-127512 on the v9.0-staging branch.

Reverts v9.0 commit db4b198c8f2b09a40175e62d9c3c7ba5d2e39a13 ("SERVER-127512 Fix index_build_killop_secondary_before_commit for in-memory suite (#54425)").

This mirrors the revert already landed on master as PR #60949 (75ff64b0db9b). This change was part of the SERVER-113184 initial sync work that has been backed out of master; the v9.0 branch is being brought back in line.

Note: the revert commit on v9.0-staging also removes jstests/replsets/initial_sync_check_initiating_set_oplog_entry.js and jstests/replsets/initial_sync_waits_for_stable_timestamp.js, which were introduced by the SERVER-125964 and SERVER-125965 backports respectively. The resulting tree is correct, but the deletions are attributed to this commit rather than to the reverts of those tickets.