python: Unset Python environment variables

This will prevent the user's environment variables from affecting host
Python, removing the need to manually override these variables.

It is also not necessary to set PYTHONPATH (when not working on target
Python packages) because the given directories are already included in
Python's search path by default.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>

lang/python/python3-package.mk

@@ -42,7 +42,6 @@ PYTHON3_VARS = \
 	__PYVENV_LAUNCHER__="/usr/bin/$(PYTHON3)" \
 	PYTHONPATH="$(PYTHON3PATH)" \
 	PYTHONDONTWRITEBYTECODE=1 \
-	PYTHONOPTIMIZE="" \
 	_python_sysroot="$(STAGING_DIR)" \
 	_python_prefix="/usr" \
 	_python_exec_prefix="/usr"