Remove api-common-protos in getting started doc

[parthea]

This getting started doc references api-common-protos which contains outdated proto files. The docs should be updated to use googleapis instead of api-common-protos.

The command should be

protoc google/cloud/vision/v1/*.proto \
    --proto_path=. \
    --python_gapic_out=/dest/

instead of

protoc google/cloud/vision/v1/*.proto \
    --proto_path=../api-common-protos/ --proto_path=. \
    --python_gapic_out=/dest/