[gdb/python] Remove Py_RETURN_{NONE,TRUE,FALSE,NOTIMPLEMENTED}

Result of:
...
$ sed -i 's/Py_RETURN_NONE/return py_none ().release ()/' gdb/python/*.{c,h}
...
with the changes in py_none reverted.

Likewise for Py_RETURN_TRUE,Py_RETURN_FALSE and Py_RETURN_NOTIMPLEMENTED.

Approved-By: Tom Tromey <tom@tromey.com>

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=34145