python-packages: Clean up build variables

python-packages: Clean up build variables

* Rename PYTHON3_PKG_SETUP_VARS to PYTHON3_PKG_BUILD_VARS, and
  PYTHON3_PKG_SETUP_DIR to PYTHON3_PKG_BUILD_PATH

  The new variable names emphasize that these values apply to the new
  build process.

* Remove PYTHON3_PKG_SETUP_ARGS set to the empty string

  These were set to override the default arguments in the old build
  process and not applicable to the new build process.

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