Remove obsolete gnome-keyring credential helper
    
    The gnome-keyring credential helper was removed in Fedora 26, but
    remained in builds for older Fedora and EL-7 releases.  No official
    release has shipped the gnome-keyring subpackage since 2.11.1-3.
    
    Obsolete it to ensure a clean upgrade path for any users who have it
    installed.  The Obsoletes can be removed after rawhide is branched for
    Fedora 29.