Simplify Option::into_flat_iter signature

[WaffleLapkin]

impl<T: IntoIterator> Option<T> {
-    pub fn into_flat_iter<A>(self) -> OptionFlatten<A>
-    where
-        T: IntoIterator<IntoIter = A>,
+    pub fn into_flat_iter(self) -> OptionFlatten<T::IntoIter> {
}

This is marked as an unresolved question (#148441, #148487 (comment)), but I cannot come up with any reason to have this generic.

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