vm.c: skip keyword argument hash duplication in OP_ENTER

The keyword argument hash passed to a method is always freshly
constructed at the call site - either by hash_new_from_regs() in
OP_SEND for inline keyword pairs, or by OP_HASH/OP_HASHCAT for
compiler-generated keyword arguments (including the **h splat case
which creates OP_HASH(0)+OP_HASHCAT). Since no caller retains a
reference to this hash, the mrb_hash_dup() was redundant.

Co-authored-by: Claude <noreply@anthropic.com>

src/vm.c

@@ -2583,9 +2583,6 @@ RETRY_TRY_BLOCK:
         kdict = mrb_nil_value();
         ci->nk = 0;
       }
-      else if (MRB_ASPEC_KEY(a) > 0 && !mrb_nil_p(kdict)) {
-        kdict = mrb_hash_dup(mrb, kdict);
-      }
       else if (!mrb_nil_p(kdict)) {
         mrb_gc_protect(mrb, kdict);
       }