build(ci): Remove unused dangerfile.js (#1280)

This dangerfile has not run since #457 (March 2023) switched CI to the
shared getsentry/github-workflows Danger workflow. That workflow executes
its own bundled dangerfile, so the repo-local one was silently ignored.

Changelog enforcement is already handled by the shared workflow (which
requires an entry for feat/fix/perf/security PRs) together with craft's
changelogPolicy: auto. Keeping a dead dangerfile that looks authoritative
was misleading, so remove it.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: runningcode