The transfer of any part of the stock of a state bank to trustees solely or primarily that they may vote the same at annual elections and stockholders' meetings - "voting trusts" as they are generally known - is expressly prohibited. A violation of this section by any bank or banks shall constitute a breach of law, and subject any such bank or banks to liquidation and forfeiture of their respective charters; provided, however, that this section shall not apply to any stock owned by an agency of the federal government.