android: Remove unused --stamp option from dex.py.

BUG=none
R=cjhopman@chromium.org

Review URL: https://codereview.chromium.org/180113011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255407 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255407