Removed irrelevant todo

UBigInt.pas

@@ -572,7 +572,6 @@ begin
     Result := TBigInt.MultiplyAbsoluteValues(A, B, A.IsNegative <> B.IsNegative);
 end;
 
-// TODO: Shift operator could be implemented with a single Move call, but I do not want to change it without test cases.
 operator shl(const A: TBigInt; const B: Integer): TBigInt;
 var
   i, j, digitShifts, bitShifts, reverseShift, len, newLength: Integer;