Removed old, unused mocks. (#1382)

(cherry picked from commit b17de011)

contracts/mocks/ForceEther.sol

-pragma solidity ^0.4.24;
-
-
-// @title Force Ether into a contract.
-// @notice  even
-// if the contract is not payable.
-// @notice To use, construct the contract with the target as argument.
-// @author Remco Bloemen <remco@neufund.org>
-contract ForceEther {
-
-  constructor() public payable { }
-
-  function destroyAndSend(address recipient) public {
-    selfdestruct(recipient);
-  }
-}

contracts/mocks/MessageHelper.sol

-pragma solidity ^0.4.24;
-
-
-contract MessageHelper {
-
-  event Show(bytes32 b32, uint256 number, string text);
-  event Buy(bytes32 b32, uint256 number, string text, uint256 value);
-
-  function showMessage(
-    bytes32 _message,
-    uint256 _number,
-    string _text
-  )
-    public
-    returns (bool)
-  {
-    emit Show(_message, _number, _text);
-    return true;
-  }
-
-  function buyMessage(
-    bytes32 _message,
-    uint256 _number,
-    string _text
-  )
-    public
-    payable
-    returns (bool)
-  {
-    emit Buy(
-      _message,
-      _number,
-      _text,
-      msg.value);
-    return true;
-  }
-
-  function fail() public {
-    require(false);
-  }
-
-  function call(address _to, bytes _data) public returns (bool) {
-    // solium-disable-next-line security/no-low-level-calls
-    if (_to.call(_data))
-      return true;
-    else
-      return false;
-  }
-
-}