core: remove .py/.pyc for Python

core: remove .py/.pyc for Python

The main Buildroot Makefile was removing *.py or *.pyc if Python 2 was
enabled, but for Python 3, this action was taken care of by a post
install target hook of python3.mk, which means it wouldn't work with
external modules (the .py/.pyc removal would be done before external
Python modules are installed).

We fix this by making the global *.py/*.pyc removal in the main
Makefile work for both Python 2 and Python 3.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>