refactor(platform): remove unnecessary pr.canMerge cases (#13914)

...latform/bitbucket-server/__snapshots__/index.spec.ts.snap

lib/platform/bitbucket-server/index.spec.ts

lib/platform/bitbucket-server/index.ts

lib/platform/gitea/__snapshots__/index.spec.ts.snap

lib/platform/gitea/index.spec.ts

lib/platform/gitea/index.ts

lib/platform/github/__fixtures__/graphql/pullrequest-1.json

lib/platform/github/__snapshots__/index.spec.ts.snap

lib/platform/github/graphql.ts

lib/platform/github/index.spec.ts

lib/platform/github/index.ts

lib/platform/github/types.ts

lib/platform/gitlab/__snapshots__/index.spec.ts.snap

lib/platform/gitlab/index.spec.ts

lib/platform/gitlab/index.ts

lib/platform/types.ts

lib/workers/pr/automerge.spec.ts

lib/workers/pr/automerge.ts

lib/workers/pr/index.spec.ts