remove 'const' from 'Option::iter'

Author: RalfJung

Diff for: library/core/src/option.rs

@@ -1338,9 +1338,8 @@ impl<T> Option<T> {
     /// assert_eq!(x.iter().next(), None);
     /// ```
     #[inline]
-    #[rustc_const_unstable(feature = "const_option", issue = "67441")]
     #[stable(feature = "rust1", since = "1.0.0")]
-    pub const fn iter(&self) -> Iter<'_, T> {
+    pub fn iter(&self) -> Iter<'_, T> {
         Iter { inner: Item { opt: self.as_ref() } }
     }