Now that `Store.PushCompose()` takes a `Distro` as argument, the rcm API
can use that function as well. This moves them both through the same
code path, reducing duplication.
Remove `PushComposeRequest()` and the corresponding struct. It was
supposed to allow composes with multiple output types and architectures,
but that was not yet implemented. Merging the two now simplifies moving
the compose queue out of the store in a future commit, which will then
tackle multi-image-type composes as well.