Get rid of unneeded writeable preferences.

        Reviewed by Anders Carlsson.

        com.apple.HIToolbox.plist and com.apple.WebProcess.plist were made writeable very
        early in WebKit2 development, before we moved a lot of functionality to UI process.
        They don't appear to be needed any more.

        Note that we do not even need to allow reading for com.apple.WebProcess.plist -
        it's read at process initialization before we enter the sandbox, and services
        have a different plist anyway.

        * WebProcess/com.apple.WebProcess.sb.in:



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@142013 268f45cc-cd09-0410-ab3c-d52691b4dbfc