Trim dead code in do_kv.

Trim dead code in do_kv.

A small piece of code in do_kv has three bugs:

- TARG could have been returned as an lvalue, so its refcount
  could be greater than 1, resulting in data getting clobbered.
  (See RT#20933 for previously fixed occurrence of this bug).

- LvTARG is refcounted, so it's buggy to just NULL it.

- TARG is returned without being initialised.

The first two bugs disappeared recently when we stopped putting
the lvalues in TARG for that op. The third remains.

However, it seems that code is never called. It can only be
called by putting NULL (not undef) on the Perl stack. I don't
see how that's possible here. The test suite never reaches that
code, so it seems it's just dead code.