WebKit: Remove video support on OS X

WebKit video support on OS X currently relies on QTKit to function
properly. This is problematic because QTKit video support in WebKit
requires linking against the WebKitSystemInterface specific to each OS X
release, thus preventing the same set of Qt binaries from interoprating
with several Mac Releases. As a workaround video support is disabled.

Back ported from: http://trac.webkit.org/changeset/151546

Task-number: QTBUG-32237

Change-Id: I780062ce5971960dcad7b51e6226dcf7cd7c907f
Reviewed-by: Michael Bruning <michael.bruning@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>