Remove peer dependency on frontend-build

[davidjoy]

So in #474, I added a peer dependency on frontend-build because that version of frontend-platform required a sufficiently high version of frontend-build or it would break if added to an MFE.

Now that we're significantly past that version and no MFEs could conceivably need it anymore (v8 of frontend-build), in my opinion we should remove the peer dependency to simplify our dependency graph, rather than continually bumping the dependency higher, which is actually incorrect and just increases our maintenance burden.