chore: remove obsolete workarounds in owlbot.py

[parthea]

The changes in this PR were validated by running these commands:

1. In a clone of googleapis/googleapis, run the following bazel build commands

bazel build //google/cloud/asset/v1:asset-v1-py
bazel build //google/cloud/asset/v1p5beta1:asset-v1p5beta1-py
bazel build //google/cloud/asset/v1p2beta1:asset-v1p2beta1-py
bazel build //google/cloud/asset/v1p1beta1:asset-v1p1beta1-py

2. Check out the branch in this PR and run the following commands to copy the generated code and run post processing

docker run --rm --user $(id -u):$(id -g)   -v $(pwd):/repo   -v $HOME/git/googleapis/bazel-bin:/bazel-bin   gcr.io/cloud-devrel-public-resources/owlbot-cli:latest copy-bazel-bin   --source-dir /bazel-bin --dest /repo

docker run --user $(id -u):$(id -g) --rm -v $(pwd):/repo -w /repo [gcr.io/cloud-devrel-public-resources/owlbot-python:latest](http://gcr.io/cloud-devrel-public-resources/owlbot-python:latest)

[parthea]

Wait for googleapis/gapic-generator-python#1804