t7003: ensure filter-branch prunes reflogs with the reftable backend

In t7003 we conditionally check whether the reflog for branches pruned by git-filter-branch(1) get deleted based on whether or not we use the "files" backend. Same as with the preceding commit, this condition was added because in its initial iteration the "reftable" backend did not delete reflogs when their corresponding ref was deleted. Since then, the backend has been aligned to behave the same as the "files" backend though, which makes this check unnecessary. Remove it. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>