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:

- app-admin/perl-cleaner-2.7::portage-stable
- dev-lang/perl-5.12.4-r1::portage-stable
- sys-devel/bc-1.06.95::portage-stable
- sys-devel/libperl-5.10.1::portage-stable
- sys-devel/make-3.82-r4::portage-stable
- sys-kernel/coreos-sources-3.17.6::coreos
- sys-libs/gdbm-1.9.1-r2::portage-stable
- virtual/dev-manager-0::portage-stable

Prune old releases, no need for them.