soas: webkit2gtk3 dropped the gtk2 plugin subppackage and now provides it.
    
    If we keep excluding this here, it excludes the entire webkit2gtk3
    package, which makes the image not compose at all. See:
    https://koji.fedoraproject.org/koji/taskinfo?taskID=36636184
    
    Signed-off-by: Kevin Fenzi <kevin@scrye.com>