Remove obsolete comment (#13850)

`is_integer_const()` does the const folding.

changelog: none

Author: y21

clippy_utils/src/lib.rs

@@ -1640,7 +1640,6 @@ pub fn is_integer_const(cx: &LateContext<'_>, e: &Expr<'_>, value: u128) -> bool
 
 /// Checks whether the given expression is a constant literal of the given value.
 pub fn is_integer_literal(expr: &Expr<'_>, value: u128) -> bool {
-    // FIXME: use constant folding
     if let ExprKind::Lit(spanned) = expr.kind {
         if let LitKind::Int(v, _) = spanned.node {
             return v == value;