gdb: remove dead code in make_pointer_type and make_reference_type

At the end of `make_pointer_type` and `make_reference_type`, GDB
updates the length of every type in the chain.  This is practically
dead code, because if we reach this point, we must have allocated a
new type.  After a new allocation, the chain contains only the
newly-created type itself.  See in `type_allocator::new_type ()`:

  type->chain = type;   /* Chain back to itself.  */

That is, we always have `ntype == ntype->chain`.  Therefore, the loop
can never be entered.  Remove it.

In `make_reference_type`, we also remove `*reftype = ntype;`, because
a few lines above the assignment was already made.  This is repeated
code.

Approved-By: Simon Marchi <simon.marchi@efficios.com>