Remove enable-webgl and enable-webaudio settings

These are weird settings. Epiphany doesn't let the user decide whether
to enable other WebKit-level features, except when it actually makes
sense to offer the user a choice of whether or not to do so. But letting
the user decide to break the web by disabling WebAudio or WebGL doesn't
make any sense. So remove these hidden settings.

Also, don't manually enable settings that are already enabled by default
in WebKitSettings.