config: Eliminate unused ppc/ppc64 configs.
    
    The history of the ppc/ppc64 configs is a convoluted one that started
    before the kernel source could support multiple machine types with a
    single image and before there was a usable 64-bit userspace.
    
    In the subsequent years, the powerpc code has been unified and we've
    released a 64-bit powerpc userspace. The "default" and "ppc64" configs
    on ppc64 are essentially identical and just waste space. The "ppc64"
    config on 32-bit ppc address a long since unnecessary corner case. Users
    who still want a 32-bit userspace with a 64-bit kernel can install and
    update it themselves using the 64-bit repository like i386/x86_64 users
    can do.
    
    This commit can be summarized thusly:
    rm config/ppc/ppc64
    rm config/ppc/ps3 (already unused)
    mv config/ppc64/ppc64 config/ppc64/default