chore(ci): turn off nightly generation_config.yaml update job (#13387)

Turn off the scheduled run for job that updates googleapis_commitish
inside generation_config.yaml. This is to stop nightly generation PRs
like #13370 to avoid
confusion.
For more context, read
[doc](https://docs.google.com/document/d/1-c-xATdvRVBilMAPEp-iuA8h85GiGcBAXg3J_Bk2Cuc/edit?tab=t.z60mo0pujx3h).

Note this does not turns down hermetic build code generation workflow:
- This
[workflow](https://github.com/googleapis/google-cloud-java/blob/main/.github/workflows/update_librarian_googleapis.yaml)
that updates googleapis commit for librarian.yaml now also updates
generation_config.yaml. So hermetic build code generation is triggered
on that PR. Instead of nightly generation PR, a generation PR like
#13385 should
contain the same contents.
- If needed, manual trigger is still possible.


For googleapis/librarian#6220

Author: zhumin8

.github/workflows/update_generation_config.yaml

@@ -15,8 +15,6 @@
 # downstream client libraries before they are released.
 name: Update generation configuration
 on:
-  schedule:
-  - cron: '0 2 * * *'
   workflow_dispatch:
 jobs:
   update-generation-config: