Stop enabling in_band_lifetimes in rustc_data_structures

There's a conversation in the tracking issue about possibly unaccepting `in_band_lifetimes`, but it's used heavily in the compiler, and thus there'd need to be a bunch of PRs like this if that were to happen.

So here's one to see how much of an impact it has.

(Oh, and I removed `nll` while I was here too, since it didn't seem needed.  Let me know if I should put that back.)

compiler/rustc_data_structures/src/sso/set.rs

@@ -75,7 +75,7 @@ impl<T> SsoHashSet<T> {
     /// An iterator visiting all elements in arbitrary order.
     /// The iterator element type is `&'a T`.
     #[inline]
-    pub fn iter(&'a self) -> impl Iterator<Item = &'a T> {
+    pub fn iter(&self) -> impl Iterator<Item = &T> {
         self.into_iter()
     }