chore(ci): remove dependency-review workflow

It requires the GitHub dependency graph, which is not usable on this
repository (the check failed on every PR with 'Dependency review is not
supported on this repository'). The dependency graph does not parse R
DESCRIPTION dependencies; it only covers GitHub Actions, where Dependabot
already proposes version bumps. Removing the workflow keeps CI green on
all PRs.

Author: Max Moldovan