SPEC: Drop unnecessary check for minor version of el7
    
    CentOS 7.5.1804 was release more than 2 months ago
    https://blog.centos.org/2018/05/centos-7-5-1804-released/
    
    So we can drop checks for minor versions of el7. They were used
    to distinguish between released and development versions and to use
    new optional features.
    
    Merges: https://pagure.io/SSSD/sssd/pull-request/3783
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>