Do not duplicate protos

[alexander-fenster]

At this moment, the gax package contains two duplicate of protos: in protos/ and in build/protos/, which is caused by some legacy and does not make any sense. We need to remove the protos/ folder and leave only build/protos from the files section in package.json. This will be a breaking change, the generated libraries we must make sure they only use protos from the build/protos/ folder.