koji_builder / mock site defaults: remove filelists option
    
    An explicit provides for /usr/libexec/platform-python was added to
    platform-python in RHEL 8.
    
    https://access.redhat.com/errata/RHSA-2024:6975
    
    This solves the original reason for needing the filelists option, so it
    should be safe to remove.  This will make bootstrap chroot creation
    faster for all builds, not just EPEL 8.
    
    https://pagure.io/releng/issue/12199
    
    Signed-off-by: Carl George <carlwgeorge@gmail.com>