CLEANUP: map/cli: always detach the backref from the list after "show map"

CLEANUP: map/cli: always detach the backref from the list after "show map"

There's no point checking the state before deciding to detach the backref
on "show map", it should always be done if the list is not empty. Note
that being empty guarantees that it's not linked into the list, and
conversely not being empty guarantees that it's in the list, hence the
test doesn't need to be performed under the lock.