spec: drop misleading provides
    
    Per repoquery done on Rawhide, there are some package depending on
    this soname:
    
        cherokee-0:1.2.104-1.fc25.x86_64
        dpsearch-0:4.54-0.8.20140109snap.fc26.x86_64
        dpsearch-devel-0:4.54-0.8.20140109snap.fc26.x86_64
        libgda-mysql-1:5.2.2-11.fc24.x86_64
        mysql-connector-odbc-0:5.3.4-6.fc26.x86_64
        nagios-plugins-mysql-0:2.2.1-2git.fc27.x86_64
        perdition-mysql-0:2.1-7.fc26.x86_64
        resiprocate-libs-0:1.10.2-26.fc25.x86_64
    
    But those are obviously builds from <= F27 (rawhide should have
    F28).  So it means there's some FTBFS anyways.  Since we don't
    provide libmysqlclient.so.18, even not as an symlink to existing
    library, there's no point in having this fake provide.