Stop creating barretenberg_headers target

[ludamad]

We don't really expect anyone to be using our headers. Namely, no one really is using us from C/C++ (sadface). If they did, we'd anyway generate them bbapi bindings just as TS and our planned rust target.

[ledwards2225]

/claudebox fix this

[AztecBot]

ClaudeBox: fix this ... workflow run