code: public boolean removeDoc(String collName, String docId) {
          if (mCollections.containsKey(collName)) {
              // remove IDs from collection
              Map<String, Map<String,Object>> collection = mCollections.get(collName);
              if (BuildConfig.DEBUG) {
                  log.debug("Removed doc: " + docId);
              }
              collection.remove(docId);
              return true;
          } else {
              log.warn("Received invalid removed msg for collection "
                      + collName);
              return false;
          }
      }
docs: Handles deleting a document in a collection.
  Override if you want to use your own collection data store.
  @param collName collection name
  @param docId document ID
  @return true if doc was deleted, false otherwise