Remove incorrect file reference in comment.

Commit b7eda3e0e moved XidInMVCCSnapshot() from tqual.c into snapmgr.c,
but follow-up commit c91560def incorrectly updated this reference.  We
could fix it, but as pointed out by Daniel Gustafsson, 1) the reader can
easily find the file that contains the definition of that function, e.g.
by grepping, and 2) this kind of reference is prone to going stale; so
let's just remove it.

Back-patch to all supported branches.

Reviewed by Daniel Gustafsson.

Discussion: https://postgr.es/m/CAPmGK145VdKkPBLWS2urwhgsfidbSexwY-9zCL6xSUJH%2BBTUUg%40mail.gmail.com

src/backend/storage/ipc/procarray.c

@@ -2171,8 +2171,7 @@ GetSnapshotDataReuse(Snapshot snapshot)
  * but since PGPROC has only a limited cache area for subxact XIDs, full
  * information may not be available.  If we find any overflowed subxid arrays,
  * we have to mark the snapshot's subxid data as overflowed, and extra work
- * *may* need to be done to determine what's running (see XidInMVCCSnapshot()
- * in heapam_visibility.c).
+ * *may* need to be done to determine what's running (see XidInMVCCSnapshot()).
  *
  * We also update the following backend-global variables:
  *		TransactionXmin: the oldest xmin of any snapshot in use in the