Don't emit gdbarch_return_value

Don't emit gdbarch_return_value

The previous patch introduced a new overload of gdbarch_return_value.
The intent here is that this new overload always be called by the core
of gdb -- the previous implementation is effectively deprecated,
because a call to the old-style method will not work with any
converted architectures (whereas calling the new-style method is will
delegate when needed).

This patch changes gdbarch.py so that the old gdbarch_return_value
wrapper function can be omitted.  This will prevent any errors from
creeping in.