protoc's Python output puts the serialized descriptor on a single line
which makes any two concurrent ateapi.proto changes conflict.
This is unfortunate as both proto and go generated code usually merges
with no conflict.
Stop checking in the *_pb2*.py files. The locust and nighthawk-ingress
images now generate them in a build stage via
benchmarking/locust/codegen/generate.sh.
Fixes#1824
