Remove deprecated 'ERC20FailedDecreaseAllowance' error (#4604)

Co-authored-by: Hadrien Croubois <hadrien.croubois@gmail.com>

Author: trevorgjohnson

Diff for: contracts/token/ERC20/ERC20.sol

@@ -41,11 +41,6 @@ abstract contract ERC20 is Context, IERC20, IERC20Metadata, IERC20Errors {
     string private _name;
     string private _symbol;
 
-    /**
-     * @dev Indicates a failed `decreaseAllowance` request.
-     */
-    error ERC20FailedDecreaseAllowance(address spender, uint256 currentAllowance, uint256 requestedDecrease);
-
     /**
      * @dev Sets the values for {name} and {symbol}.
      *