tests/compose: Drop FCOS postprocess scripts

Now that we've bumped to the latest FCOS commit for compose tests, one
thing that came up was that our compose tests never actually included
FCOS overlays in the compose the way cosa does.

This then cause compose failures because one of the postprocess scripts
expects those files there.

Let's just nuke all postprocess scripts here to work around this. I
initially wanted to import the overlay logic from cosa, but overlays
only work in unified core mode, and sadly we still want some coverage in
non-unified mode until that's fully dropped.

And anyway, we also already do a proper `cosa build` in the vmcheck
branch of CI so it's not like we're losing that coverage.

Down the line though, I think this is a good argument for folding the
overlay dirs into rpm-ostree more natively as discussed here:

https://github.com/coreos/coreos-assembler/pull/639#issuecomment-534713737

tests/compose.sh

@@ -126,6 +126,7 @@ EOF
 import sys, json
 y = json.load(sys.stdin)
 y["repos"] = ["cache"]
+y["postprocess"] = []
 y.pop("lockfile-repos", None)
 json.dump(y, sys.stdout)' < manifest.json > manifest.json.new
   mv manifest.json{.new,}