chore(backend): clean up backend code generation

[Bobgy]

Description of your changes:
Changes:

• Prefer downloading required tools than building via bazel.
• Clean up proto unused imports (but I found error.proto imports are required).
• Remove licenses from generated code, because that's no longer required.
• Remove BUILD.bazel files that are no longer used.

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[Bobgy]

/cc @NikeNano

[Bobgy]

Changes to remove importing error package is reverted, because after that, I get the following error during swagger generation:

/tmp/tmp.nP3pFaOTw6/swagger_linux_amd64 generate client -f /home/gongyuan_google_com/kfp/pipelines/backend/api/swagger/pipeline.swagger.json -A pipeline --principal models.Principal -c pipeline_client -m pipeline_model -t /home/gongyuan_google_com/kfp/pipelines/backend/api/go_http_client
2021/02/09 09:19:23 validating spec /home/gongyuan_google_com/kfp/pipelines/backend/api/swagger/pipeline.swagger.json
The swagger spec at "/home/gongyuan_google_com/kfp/pipelines/backend/api/swagger/pipeline.swagger.json" is invalid against swagger specification 2.0. see errors :
- invalid ref ".api.Status"
- ```

[Bobgy]

OK, if tests pass, I think this is ready to merge.

Then the follow up PRs to remove bazel should contain minimal changes: either #4906 or #5115

[Bobgy]

/assign @NikeNano
Can you help me review and LGTM?

[NikeNano]

/LGTM

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Bobgy, NikeNano

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