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.