refactor: remove branch functions from platform (#6676)

lib/manager/npm/post-update/index.ts

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

lib/platform/azure/index.ts

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

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

lib/platform/bitbucket-server/index.ts

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

lib/platform/bitbucket/index.spec.ts

lib/platform/bitbucket/index.ts

lib/platform/common.ts

lib/platform/gitea/index.spec.ts

lib/platform/gitea/index.ts

lib/platform/github/index.ts

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

lib/platform/gitlab/index.spec.ts

lib/platform/gitlab/index.ts

lib/workers/branch/automerge.spec.ts

lib/workers/branch/automerge.ts

lib/workers/branch/index.spec.ts

lib/workers/branch/index.ts