Remove unused --defines flag from python code generation scripts

This patch removes the --defines flag currently passed to our python
code generators, which appears to be completely unused.

Review URL: https://codereview.chromium.org/394153002

git-svn-id: svn://svn.chromium.org/blink/trunk@178321 bbb929c8-8fbe-4397-9dbb-9b2b20218538