refs: update comment.

REF_IS_PRUNING is right below this comment, so it clearly does not belong in
this comment. This was apparently introduced in commit 5ac95fee (Nov 5, 2017
"refs: tidy up and adjust visibility of the `ref_update` flags").

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs/files-backend.c

@@ -16,8 +16,7 @@
  * This backend uses the following flags in `ref_update::flags` for
  * internal bookkeeping purposes. Their numerical values must not
  * conflict with REF_NO_DEREF, REF_FORCE_CREATE_REFLOG, REF_HAVE_NEW,
- * REF_HAVE_OLD, or REF_IS_PRUNING, which are also stored in
- * `ref_update::flags`.
+ * or REF_HAVE_OLD, which are also stored in `ref_update::flags`.
  */
 
 /*