[3.12] gh-76106: Remove the cleanup lock in test_socket (GH-110539) (GH-110699)

It does not already work (because it locks only addCleanup(), not doCleanups()),
and it is no longer needed since the clean up procedure waits for all test threads to join.
(cherry picked from commit f27b83090701b9c215e0d65f1f924fb9330cb649)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>