coreos-firmware: remove coreos-sources from runtime dependencies

Fixes commit 3f028060 which mistakenly added coreos-sources as both
build and run-time dependencies. This was missed initially because there
are exclude rules that remove /usr/src from production images but it
does needlessly slow down the build and pulls in extraneous kernel build
dependencies like perl.

Prune old releases, no need for them.
Merge pull request #1010 from marineam/firmware

coreos-firmware: remove coreos-sources from runtime dependencies