gdb: remove decref_target

The decref_target function is not really needed.  Calling
target_ops::decref will just redirect to decref_target anyway, so why
not just rename decref_target to target_ops::decref?

That's what this commit does.

It's not exactly renaming to target_ops::decref, because the decref
functionality is handled by a policy class, so the new name is now
target_ops_ref_policy::decref.

There should be no user visible change after this commit.