tools/cephfs_mirror: Remove sync-stat omap metrics on peer remove and mirror disable

Remove persisted sync-stat omap entries when a peer is removed or
mirroring is disabled for a filesystem, since those peer UUIDs are not
reused. Preserve omap on internal blocklist/failure restarts so sync
can resume where it left off.

Also update the relevant documentation

Fixes: https://tracker.ceph.com/issues/78464
Signed-off-by: Kotresh HR <khiremat@redhat.com>