Remove unused parts of WebInputEventFactoryWin.

Almost everything seems to go through the Aura path now.

BUG=363455

Review URL: https://codereview.chromium.org/235883008

git-svn-id: svn://svn.chromium.org/blink/trunk@171521 bbb929c8-8fbe-4397-9dbb-9b2b20218538