Pipelines: Backend dependency cleanup - remove bazel

[jingzhang36]

Will help to reduce backend build time and easy dependency version bumping

[Bobgy]

All high priority tasks have been done.

Right now, bazel is only used to generate swagger json definitions from proto and that's a offline task.
Contributions welcomed on finishing the last step to remove bazel, but I believe it's not high priority because generated code are commited in the repo.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[Bobgy]

For the last issue, I just noticed we had a makefile for generating API swagger definitions from proto (before we used bazel):
https://github.com/kubeflow/pipelines/blob/2f3a686e545b671d3d1ce96bbcc07b18cb905a87/backend/api/Makefile

We may use it as a reference when migrating bazel based generation script back.

[Bobgy]

A quick recap of remaining issue:

• [ ] Remove bazel usage from https://github.com/kubeflow/pipelines/blob/master/backend/api/generate_api.sh

jingzhang36 had an WIP PR for removing bazel from proto generation script: https://github.com/kubeflow/pipelines/pull/4393

but it went into the wrong direction, because we need the generated code checked in the repo, rather than during docker build step.

[Bobgy]

/cc @NikeNano
this is the tracker issue for removing bazel, in case you didn't know it.

[NikeNano]

Since we have solved the last out standing point here I believe this can be removed. https://github.com/kubeflow/pipelines/issues/3250#issuecomment-744114178 @Bobgy

[Bobgy]

A long time headache is over : )

[NikeNano]

Succes :)