Revert some unneeded code changes from environment list

I neglected to read https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6303#note_92246845 before merging gitlab-org/gitlab-ce!21018 and it turns out the can_deploy property is no longer needed because it can be inferred from the existence or non-existence of other manual action properties.

The code in question is harmless to leave in, but it is now a vestigial part of an earlier implementation that can be removed. This MR corrects that oversight.