:seedling: Remove `cmd/kcp-core` binary

[embik]

Summary

#2795 introduced cmd/kcp-core as a means to differentiate between the "core" extended apiserver and the core+tmc features (called "kcp"). However, when the project moved to community governance, "kcp" itself became the designated name for the "core" component and tmc moved to an external repository.

I'd propose we remove kcp-core, since it never served its intended purpose and probably won't either (if we ever revive transparent multi-cluster, I suspect we'll be naming the binary/project tmc).

Related issue(s)

Fixes #

Release Notes

Remove `kcp-core` binary (`kcp` remains unchanged)