remove unused y0, clean-up pre-conditions

Author: hoytech

src/libraries/CurveExtrasLib.sol

@@ -5,12 +5,8 @@ import {Math} from "openzeppelin-contracts/utils/math/Math.sol";
 
 library CurveExtrasLib {
     /// @dev EulerSwap derivative helper function to find the price after a swap
-    /// Pre-conditions: 0 < x <= x0, 1 <= {px,py} <= 1e36, {x0,y0} <= type(uint112).max, c <= 1e18
-    function df_dx(uint256 x, uint256 px, uint256 py, uint256 x0, uint256 y0, uint256 c)
-        internal
-        pure
-        returns (int256)
-    {
+    /// Pre-conditions: 0 < x <= x0 <= type(uint112).max, 1 <= {px,py} <= 1e36, c <= 1e18
+    function df_dx(uint256 x, uint256 px, uint256 py, uint256 x0, uint256 c) internal pure returns (int256) {
         uint256 r = Math.mulDiv(x0 * x0 / x, 1e18, x, Math.Rounding.Ceil);
         return -int256(px * (c + (1e18 - c) * r / 1e18) / py);
     }