fix(v1beta2): enable self signed jwt for grpc

[nicain]

Remove Owlbot v1beta2 customization that passes default scopes as user scopes, so that the auth library does not use the self-signed JWT flow.

Fixes #174

[nicain]

@busunkim96 Are there special Labels necessary to engage owlbot for this change?

[google-cla]

All (the pull request submitter and all commit authors) CLAs are signed, but one or more commits were authored or co-authored by someone other than the pull request submitter.

We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that by leaving a comment that contains only @googlebot I consent. in this pull request.

Note to project maintainer: There may be cases where the author cannot leave a comment, or the comment is not properly detected as consent. In those cases, you can manually confirm consent of the commit author(s), and set the cla label to yes (if enabled on your project).

ℹ️ Googlers: Go here for more info.

[parthea]

@nicain I'm not aware of any labels. I ran owlbot locally using these steps and pushed to this branch.

# pull the latest docker image for the owlbot-cli
docker pull gcr.io/repo-automation-bots/owlbot-cli:latest

# run the owlbot-cli container to pull changes from googleapis/googleapis-gen
docker run --rm --user $(id -u):$(id -g) -v $(pwd):/repo -w /repo gcr.io/repo-automation-bots/owlbot-cli:latest copy-code

# pull the latest python post processing image
docker pull gcr.io/repo-automation-bots/owlbot-python:latest

# run the python post processing container to synchronize templated files and format the code
docker run --user $(id -u):$(id -g) --rm -v $(pwd):/repo -w /repo gcr.io/repo-automation-bots/owlbot-python:latest

@googlebot I consent.

[google-cla]

All (the pull request submitter and all commit authors) CLAs are signed, but one or more commits were authored or co-authored by someone other than the pull request submitter.

We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that by leaving a comment that contains only @googlebot I consent. in this pull request.

Note to project maintainer: There may be cases where the author cannot leave a comment, or the comment is not properly detected as consent. In those cases, you can manually confirm consent of the commit author(s), and set the cla label to yes (if enabled on your project).

ℹ️ Googlers: Go here for more info.

[parthea]

@googlebot I consent.