lib: drop "-2" flag for "py.exe" launcher

Now that node-gyp supports both Python 2 and Python 3,
We don't need to explicitly try to find only Python 2.

Fixes: https://github.com/nodejs/node-gyp/issues/2130
Refs: https://docs.python.org/3/using/windows.html#launcher
PR-URL: https://github.com/nodejs/node-gyp/pull/2131
Reviewed-By: Christian Clauss <cclauss@me.com>
Reviewed-By: Rod Vagg <rod@vagg.org>