Mk/bsd.default-versions.mk: Remove PYTHON3_DEFAULT and use PYTHON_DEFAULT for Python 3.x

• Remove unnecessary PYTHON3_DEFAULT
• Use PYTHON_DEFAULT for Python 3.x and remove 2.7 from PYTHON_DEFAULT
• Use PYTHON2_DEFAULT for Python 2.7

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.