Remove unnecessary code

Bug: 19313118

As part of the "better error reporting for Webview" work, a new public
API was defined for MNC release to report blocked loads. However,
we decided to use WebChromeClient.onConsoleMessage for this case.
Removing the API.

Change-Id: I1a599385f1ecdd10ba5a774b0b2a6b9f4bdcbd95