ci: remove LEAF_JOBS parameter from dispatcher job (#1980)

## Summary

- Removes the `LEAF_JOBS` string parameter from the dispatcher Jenkins
job (`proj-jjb.yaml`)
- Removes the corresponding filtering logic in `Jenkinsfile.dispatcher`
— the dispatcher now always fans out to all defined leaf jobs
- Updates `ci-overview.md` to drop documentation of the removed
parameter

## Why

The `LEAF_JOBS` parameter listed leaf job names in its default value.
When a new leaf job was added on `main`, older release branches would
fail at dispatcher runtime because the new name wasn't in their copy of
the parameter default — triggering the unknown-entry validation error.
Since the parameter isn't actively used, removing it is simpler than
maintaining the list across branches.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Documentation**
* Updated CI documentation to reflect that dispatcher runs execute all
available build jobs.

* **Bug Fixes**
* Removed the option to selectively skip individual build jobs during
dispatcher runs.
* Simplified dispatcher configuration by removing the unused
job-selection parameter.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: NirWolfer <nwolfer@nvidia.com>