gtkwin: remove dead code in cut buffer handling.

Commit d851df486f066b3 deleted a #if / #else / #endif on the grounds
that the condition would now always be true, without also deleting the
code inside the #else. Happily, the then-branch ended with a return,
so it was a benign mistake - the erroneously left-in else-clause code
was unreachable. But now Coverity has pointed it out, let's remove it.

unix/gtkwin.c

@@ -2840,8 +2840,6 @@ static char *retrieve_cutbuffer(GtkFrontend *inst, int *nbytes)
         ptr = 0;
     }
     return ptr;
-    *nbytes = 0;
-    return NULL;
 }
 
 #endif /* NOT_X_WINDOWS */