PG: fix cached_removed_snaps bug in PGPool::update after map gap

PG: fix cached_removed_snaps bug in PGPool::update after map gap

5798fb3bf6d726d14a9c5cb99dc5902eba5b878a actually made 15943 worse
by always creating an out-of-date cached_removed_snaps value after
a map gap rather than only in the case where the the first map after
the gap did not remove any snapshots.

Introduced: 5798fb3bf6d726d14a9c5cb99dc5902eba5b878a
Fixes: http://tracker.ceph.com/issues/15943
Signed-off-by: Samuel Just <sjust@redhat.com>
(cherry picked from commit 5642e7e1b3bb6ffceddacd2f4030eb13a17fcccc)