static async cleanStoredManifest(manifestId) {
    const muxer = new shaka.offline.StorageMuxer();
    await muxer.init();
    const activeHandle = await muxer.getActive();
    const uri = shaka.offline.OfflineUri.manifest(
        activeHandle.path.mechanism,
        activeHandle.path.cell,
        manifestId);
    await muxer.destroy();
    const storage = new shaka.offline.Storage();
    await storage.remove(uri.toString());
  }
Removes all of the contents for a given manifest, statelessly.

@param {number} manifestId
@return {!Promise}