Remove reference to redhat-rpm-templates
    
    Page SourceURL.adoc referred to package redhat-rpm-templates,
    which does not exist.
    Apparently, it was intended to be part of the following pull request,
    but was dropped:
    
    https://src.fedoraproject.org/rpms/redhat-rpm-config/pull-request/51
    
    Apparently, the referred macros are currently only available
    in these Guidelines.
    Remove the package reference.
    
    Fixes #1215