Remove Python API checker defines

The GCC plugin that implements the Python API checker does not appear
to really be maintained.  And, as far as I know, it never really
worked for C++ code anyway.  Considering those factors, and that no
one has tried to run it in years, I think it's time to remove the
macros from the gdb source.

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