Do not cache deprecated PYTHON_INCLUDE_PATH.

This variable has been deprecated for years and its presence in
the cache can lead to frustration.  If a user removes the variable
PYTHON_INCLUDE_DIR from their cache (e.g. while trying to debug a
build), then cmake will use PYTHON_INCLUDE_PATH instead of searching
the system for Python.h like it should.  Removing PYTHON_INCLUDE_PATH
from the cache fixes this problem.

Change-Id: I23e5abf8655f31f19daa144489983d2adf206aad
Merge topic 'uncache-python-include-path' into master

a901d1e2 Do not cache deprecated PYTHON_INCLUDE_PATH.