docs(backend): Remove deprecated v2 compatibility mode docs

[droctothorpe]

Description of your changes:
This PR just removes some deprecated documentation from the backend/src/v2 README file.

Checklist:

• The title for your pull request (PR) should follow our title convention. Learn more about the pull request title convention used in this repository.

[chensun]

/lgtm

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chensun

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[hbelmiro]

@droctothorpe can you sign off your commits?

[droctothorpe]

@droctothorpe can you sign off your commits?

Fixed. Sorry about that!

[google-oss-prow]

@droctothorpe: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
kubeflow-pipelines-samples-v2	6ffd2b9	link	false	/test kubeflow-pipelines-samples-v2
kubeflow-pipeline-upgrade-test	6ffd2b9	link	false	/test kubeflow-pipeline-upgrade-test

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository. I understand the commands that are listed here.

[hbelmiro]

/lgtm