spec: Drop pathfix.py usage
    
    There's only one remaining ambiguous python shebang in qemu.git, and
    it's not for anything we care about.
    
    Also redhat-rpm-config brp-mangle-shebangs added in 2018 will do
    /usr/bin/env python3 -> /usr/bin/python3 automatically for us
    
    Signed-off-by: Cole Robinson <crobinso@redhat.com>