ci: Remove unit tests and doc tests from kokoro presubmit. (#2195)

* The message is a commit message, not a message to a user from an AI agent. Therefore, it should be output as is.

Output:
Remove Kokoro presubmit for unit, docs, and coverage.

This commit removes the Kokoro presubmit configuration that runs
`unit_noextras`, `unit`, `cover`, `docs`, and `docfx` nox sessions.
These checks are already performed by GitHub Actions, making the
Kokoro configuration redundant.

The change involves removing the `NOX_SESSION` environment variable
definition from `.kokoro/presubmit/presubmit.cfg`.

* Update presubmit.cfg

* Delete .kokoro/presubmit/presubmit.cfg

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Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

Author: chalmerlowe