app/views/shared/issuable/_filter.html.haml can be removed safely
app/views/shared/issuable/_filter.html.haml was (probably unintentionally) re-introduced in !21438 (merged) (squashed: fbb4f15d, original: 84e00336), while it has been removed in !22641 (merged).
That partial template does not seem required from any other templates as far as I inspected !21438 (merged) through the squashed commit (fbb4f15d) and the related original commit (84e00336). We could remove that file safely now.
The first comment about that was at https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23301#note_126927734