android envsetup: Make passing --target-arch an error.

This is to make sure I found all the bots that used to pass this. I'll
completely remove the flag in a day or two.

Also remove the function for printing usage information. I accidentally removed
processing for --help in r252034, but having a --help flag for printing --help
as only supported flag is a bit silly anyways, so get rid of this.

(Depends on https://codereview.chromium.org/170783015/
and https://chrome-internal-review.googlesource.com/#/c/155292/)

BUG=330631
R=yfriedman@chromium.org

Review URL: https://codereview.chromium.org/173003003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252166 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252166