Remove FHEPayment contract and create FHEGasLimit.sol

[immortal-tofu]

Payment won't be used anymore.
The idea is to remove the FHEPayment.sol and instead of create a minimized version called FHEGasLimit.sol.

This contract is not part of core components anymore, but rather an extension to call in TFHEExecutor similar to:

contract TFHEExecutorWithLimit is TFHEExecutor {
    function fheAdd(uint256 lhs, uint256 rhs, bytes1 scalarByte) public virtual override returns (uint256 result) {
        fheGasLimit.consumeGasForFheAdd(msg.sender, lhsType, scalar);
        result = super.fheAdd(lhs, rhs, scalarByte);
    }
}

[jatZama]

Done in #211
With a caveat: for the moment we agreed on Slack to keep the call to FHEGasLimit by default inside the original TFHEExecutor contract. For two reasons:
1/ it is critical to avoid DDOS attack to keep some limitation on number of FHE ops
2/ and otherwise we would have to maintain 4 versions of almost same contract: TFHEExecutor , TFHEExecutorWithLimit, TFHEExecutorWithEvents and TFHEExecutorWithEventsWithLimit.