Remove unused unit_list_compare

I'm not sure since when this has been unused but I've only started seeing the
warnings recently.

client/mapctrl_common.cpp

@@ -374,41 +374,3 @@ void update_line(int canvas_x, int canvas_y)
     break;
   };
 }
-
-/**
-   We sort according to the following logic:
-
-   - Transported units should immediately follow their transporter (note that
-     transporting may be recursive).
-   - Otherwise we sort by ID (which is what the list is originally sorted
- by).
- */
-static int unit_list_compare(const void *a, const void *b)
-{
-  const struct unit *punit1 = *(struct unit **) a;
-  const struct unit *punit2 = *(struct unit **) b;
-
-  if (unit_transport_get(punit1) == unit_transport_get(punit2)) {
-    // For units with the same transporter or no transporter: sort by id.
-    // Perhaps we should sort by name instead?
-    return punit1->id - punit2->id;
-  } else if (unit_transport_get(punit1) == punit2) {
-    return 1;
-  } else if (unit_transport_get(punit2) == punit1) {
-    return -1;
-  } else {
-    /* If the transporters aren't the same, put in order by the
-     * transporters. */
-    const struct unit *ptrans1 = unit_transport_get(punit1);
-    const struct unit *ptrans2 = unit_transport_get(punit2);
-
-    if (!ptrans1) {
-      ptrans1 = punit1;
-    }
-    if (!ptrans2) {
-      ptrans2 = punit2;
-    }
-
-    return unit_list_compare(&ptrans1, &ptrans2);
-  }
-}