qa/tests: fix POOL_FULL ignorelist pattern in upgrade tests

\(POOL_FULL\) only matches the "Health check failed/update" cluster-log
format (code wrapped in parens). It misses the "Health check cleared"
format, where the code is not parenthesized (see src/mon/Monitor.cc),
so a pool clearing its full state mid-upgrade-test was not ignorelisted.
Drop the escaped parens so the pattern matches both forms, consistent
with other bare entries already in this file (OSD_ROOT_DOWN,
MDS_INSUFFICIENT_STANDBY).

Fixes: https://tracker.ceph.com/issues/78149
Signed-off-by: Yuri Weinstein <yweinste@redhat.com>