raid5-ppl: don't resync after rebuild

The check for degraded array is unnecessary and causes a resync to be
performed after ppl recovery and rebuild when restarting an array during
rebuilding after unclean shutdown.

Signed-off-by: Artur Paszkiewicz <artur.paszkiewicz@intel.com>
Signed-off-by: Shaohua Li <shli@fb.com>