Remove Modular, Atomic, Docker and updates compose types
Modular, Atomic and Docker composes have not existed for like
four years at this point, so there's almost no point in fedfind
supporting them any more. Any obscure corner case that might
want to do something with old PDC data for those composes, or
something, can use an old version of fedfind.
The updates and updates-testing composes still exist, but never
produce images any more, so instead of handling them, we'll just
treat them as explicitly unsupported.
We also update some comments and documentation around this, and
can simplify a few bits of code now they no longer need to worry
about these compose types.
Signed-off-by: Adam Williamson <awilliam@redhat.com>