Remove some probably dead code in EventHandler::handleDrag.

This may have been used for AppKit integration at some point in the
distant past. Two checks for the left mouse button have been moved
up a bit higher and combined, since drags cannot be triggered by
any other button.

BUG=none

Review URL: https://codereview.chromium.org/726703002

git-svn-id: svn://svn.chromium.org/blink/trunk@185389 bbb929c8-8fbe-4397-9dbb-9b2b20218538