Fix 500 when selecting a mirror user
When selecting a mirror user, we do a permissions check, which means
that @users
is an array, not an ActiveRecord relation.
We also can't use Array#delete
, because it uses Object#equal?
, while
we need to be able to take advantage of ActiveRecord objects overriding
#eql?
and #hash?
(as Array#uniq
uses a hash internally).
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/29316.