ERC721 transfers should not emit Approval events

[AnAllergyToAnalogy]

As per the ERC721 standard, transfers shouldn't emit an Approval event. It's implied by the Transfer event being emitted.

This means the emitted Approval events are unwanted/unexpected behavior, and it also means literally every transfer wastes about 2000 gas for no reason.

    /// @dev This emits when the approved address for an NFT is changed or
    ///  reaffirmed. The zero address indicates there is no approved address.
    ///  When a Transfer event emits, this also indicates that the approved
    ///  address for that NFT (if any) is reset to none.
    event Approval(address indexed _owner, address indexed _approved, uint256 indexed _tokenId);