Mk/Uses/python.mk: Remove PYTHON3_DEFAULT and use PYTHON_DEFAULT instead
- Remove unnecessary PYTHON3_DEFAULT
- Replace PYTHON3_DEFAULT with PYTHON_DEFAULT
Regarding DEFAULT_VERSIONS, both python and python3 point to the same value
which makes python3 a duplicate. And we do not really support python=2.7.
Therefore, use python for Python 3.x and python2 for legacy Python 2.7.