Removed a bunch of unused import statements found after running pylint

BUG=271470
NOTRY=true

Review URL: https://codereview.chromium.org/110903003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240720 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240720