mon: don't require OSD W for MRemoveSnaps

mon: don't require OSD W for MRemoveSnaps

Use ability to execute "osd pool rmsnap" command
as a signal that the client should be permitted
to send MRemoveSnaps too.

Note that we don't also require the W ability,
unlike Monitor::_allowed_command -- this is slightly
more permissive handling, but anyone crafting caps
that explicitly permit "osd pool rmsnap" needs to
know what they are doing.

Fixes: #13777
Signed-off-by: John Spray <john.spray@redhat.com>