Remove override for `assignees` association in `app/models/concerns/deprecated_assignee.rb`

There is currently an override for assignees association in app/models/concerns/deprecated_assignee.rb

This is an override for a has_many through: association here. Overriding through: associations can turn out to be problematic, as described in https://docs.gitlab.com/ee/development/gotchas.html#do-not-override-has_many-through-or-has_one-through-associations and needs to be removed.

References: #424536 (closed)