Remove build flags that no longer exist
    
    The 'mimic' codec was used by MSN messenger in prehistoric times, and
    was removed [1]. The 'nas' and 'xvid' plugins were removed because
    they were never ported to GStreamer 1.0 [2].
    
    The 'mpg123' plugin was moved from -bad to -ugly [3].
    
    No need to issue a new build just for this.
    
    [1] gst-plugins-bad commit ad661999ad2c79a5
        https://bugzilla.gnome.org/show_bug.cgi?id=597616
    
    [2] gst-plugins-bad commit 9b5de053995488d5
        https://bugzilla.gnome.org/show_bug.cgi?id=774530
    
    [3] gst-plugins-bad commit 08d8aefcdaaf89ec
        https://bugzilla.gnome.org/show_bug.cgi?id=719849