chore: remove unused modifier

Signed-off-by: Tomás Migone <[email protected]>

Author: tmigone

packages/horizon/contracts/staking/HorizonStakingExtension.sol

@@ -30,14 +30,6 @@ contract HorizonStakingExtension is HorizonStakingBase, IHorizonStakingExtension
     using TokenUtils for IGraphToken;
     using PPMMath for uint256;
 
-    /**
-     * @dev Checks that the sender is the L2GraphTokenGateway as configured on the Controller.
-     */
-    modifier onlyL2Gateway() {
-        require(msg.sender == address(_graphTokenGateway()), "ONLY_GATEWAY");
-        _;
-    }
-
     /**
      * @dev Check if the caller is the slasher.
      */