Remove stale spec patches for closed tickets

PYTHON-2673 (connection pinning for load balanced clusters), PYTHON-3712
(deprecate sharded-replicaset topology type), and PYTHON-4261 (server
selection log line noise) are all closed/fixed. Their patch files were
never removed, causing PYTHON-2673 to fail to apply on every spec sync
run since the underlying test files moved on after the tickets were
resolved.

Author: aclark4life