Get rid of the python_arch variable.

It's only used for pyauto, which is on the way out.

BUG=277641,224072,177664
NOTRY=true

Review URL: https://codereview.chromium.org/121013002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242551 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242551