Don't build unnecessary flatbuffers/flatcc targets (#2466)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2466

We're currently building unnecessary pieces of the flatbuffers and flatcc submodules. I had tried disabling them before for flatbuffers, but didn't do it the right way.

Now do it the right way, saving us some time during the build.

Reviewed By: larryliu0820, GregoryComer

Differential Revision: D54966722

fbshipit-source-id: 3ceffeab893680fc755f670e8ade57b8605f818d