Remove redundant "pets don't attack each other" check

There were two copies of this check; take the one that looks like
vanilla, but I moved it up slightly to be outside the "Grudge patch"
comment.

src/mon.c

@@ -2396,18 +2396,15 @@ mm_aggression(
     struct permonst *ma, *md;
     ma = magr->data;
     md = mdef->data;
-    /* Don't allow pets to fight each other. */
+
+    /* don't allow pets to fight each other */
     if (magr->mtame && mdef->mtame)
         return 0;
 
     /* Grudge patch. */
     /* Put one-way aggressions below here, and two-way aggressions in
      * mm_2way_aggression. */
 
-    /* don't allow pets to fight each other */
-    if (magr->mtame && mdef->mtame)
-        return 0;
-
     /* supposedly purple worms are attracted to shrieking because they
        like to eat shriekers, so attack the latter when feasible */
     if ((ma == &mons[PM_PURPLE_WORM] || ma == &mons[PM_BABY_PURPLE_WORM])