Remove handling of ENTER key from WebView.

WebView was (incorrectly) handling the ENTER key as if it could start
a long press like CENTER key does.  Separate the two behaviors.  Now,
the ENTER key does not do anything unless it is part of a textfield or
the DOM otherwise handles it.

As a result, I was able to remove some orphaned code.  Before this
change, pressing ENTER would play a click noise, and animate the
focus ring as if its link were being followed, but do nothing. In
WebViewCore, do not pass the ENTER key back to the Activity.