fix: add docfx to the presubmit configuration and delete docs-presubm…

…it (#1995)

* adjusts location of checks related to docfx/docs

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

---------

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

.kokoro/presubmit/presubmit.cfg

@@ -3,5 +3,5 @@
 # Disable system tests.
 env_vars: {
     key: "NOX_SESSION"
-    value: "unit_noextras unit cover docs"
+    value: "unit_noextras unit cover docs docfx"
 }