testsuite/gcc.dg/uninit-pred-9_b.c:23: Un-xfail for MMIX

In a recent all-target test-round investigating XPASSes for
this file, I noticed this line XPASSing for MMIX.  From the
commit history it's obvious it was left out from related
target-xfail tweaks, now the last target xfailing a bogus
warning for this line.

	* gcc.dg/uninit-pred-9_b.c: Remove xfail for MMIX from line 23.

gcc/testsuite/gcc.dg/uninit-pred-9_b.c

@@ -20,7 +20,7 @@ int foo (int n, int l, int m, int r)
       blah(v); /* { dg-bogus "uninitialized" "bogus warning" { xfail *-*-* } } */
 
   if ( (n <= 8) &&  (m < 99)  && (r < 19) )
-      blah(v); /* { dg-bogus "uninitialized" "pr101674" { xfail mmix-*-* } } */
+      blah(v); /* { dg-bogus "uninitialized" "pr101674" } */
 
   return 0;
 }