code: public void deleteColumn(String storeName, String rowKey, String columnName) {
          m_columnDeletes.add(new ColumnDelete(storeName, rowKey, columnName));
      }
docs: Add an update that will delete the column for the given store, row key, and column
  name. If a column update exists for the same store/row/column, the results are
  undefined when the transaction is committed.
  
  @param storeName Name of store that owns row.
  @param rowKey    Row key in string form.
  @param colName   Column name in string form.