refactor: remove git-submodules datasource (#9128)

lib/datasource/api.ts

lib/datasource/git-submodules/index.spec.ts

lib/datasource/git-submodules/index.ts

lib/manager/git-submodules/extract.spec.ts

lib/manager/git-submodules/extract.ts

lib/manager/git-submodules/update.ts

lib/workers/branch/get-updated.spec.ts

lib/workers/branch/get-updated.ts

...epository/process/lookup/__snapshots__/index.spec.ts.snap

lib/workers/repository/process/lookup/index.spec.ts

lib/workers/repository/process/lookup/index.ts