Remove bits of dead code identified by coverage tests

These lines could were not hit by our tests, and examination
of the code reveals that they can't ever be hit.

glib/gslist.c

@@ -316,16 +316,8 @@ g_slist_insert (GSList   *list,
       tmp_list = tmp_list->next;
     }
 
-  if (prev_list)
-    {
-      new_list->next = prev_list->next;
-      prev_list->next = new_list;
-    }
-  else
-    {
-      new_list->next = list;
-      list = new_list;
-    }
+  new_list->next = prev_list->next;
+  prev_list->next = new_list;
 
   return list;
 }