refactor build generator

[DavidKorczynski]

https://github.com/google/oss-fuzz-gen/tree/main/experimental/c-cpp is used for generating builds and also has legacy code for harness generation and evaluation. We should:

• remove the harness generation logic
• focus the logic only on build generation
• make a seamless integration with OFG harness generation logic

[oliverchang]

thanks! can we close this now?