ci: drop the renovate rebase dispatch

[BryanFRD]

Renovate now runs as a CronJob in the cluster (FerrLabs/Infra#424), and FerrLabs/.github#339 removes renovate.yml and reusable-renovate-dispatch.yml. The "rebase now" dispatch has nothing left to trigger: a ticked checkbox is handled by the next in-cluster run (every 2 h).

Merge after FerrLabs/.github#339.

[ferrfleet]

Clean deletion. Searched the repo: the only other mentions of the rebase dispatch are historical CHANGELOG entries (#175, #191), and renovate.json just extends github>FerrLabs/.github, which the in-cluster run still reads. Nothing else calls reusable-renovate-dispatch.yml.

Nit: the merge ordering in the description is stricter than it needs to be. Removing the caller is safe at any time; it is the reverse (deleting the reusable workflow first) that would break things, so this does not have to wait on FerrLabs/.github#339.

One thing worth checking outside the diff: if dispatch is listed as a required status check in branch protection on main, deleting the workflow leaves that check permanently unreported and every PR stuck. I can't read branch protection from here, so this is unverified.