Be more lax in .into_iter() suggestion when encountering Iterator methods on non-Iterator

```
error[E0599]: no method named `map` found for struct `Vec<bool>` in the current scope
  --> $DIR/vec-on-unimplemented.rs:3:23
   |
LL |     vec![true, false].map(|v| !v).collect::<Vec<_>>();
   |                       ^^^ `Vec<bool>` is not an iterator
   |
help: call `.into_iter()` first
   |
LL |     vec![true, false].into_iter().map(|v| !v).collect::<Vec<_>>();
   |                       ++++++++++++
```

We used to provide some help through `rustc_on_unimplemented` on non-`impl Trait` and non-type-params, but this lets us get rid of some otherwise unnecessary conditions in the annotation on `Iterator`.

Author: estebank

core/src/iter/traits/iterator.rs

@@ -34,15 +34,6 @@ fn _assert_is_object_safe(_: &dyn Iterator<Item = ()>) {}
         _Self = "core::ops::range::RangeToInclusive<Idx>",
         note = "you might have meant to use a bounded `RangeInclusive`"
     ),
-    on(
-        _Self = "[]",
-        label = "`{Self}` is not an iterator; try calling `.into_iter()` or `.iter()`"
-    ),
-    on(_Self = "&[]", label = "`{Self}` is not an iterator; try calling `.iter()`"),
-    on(
-        _Self = "alloc::vec::Vec<T, A>",
-        label = "`{Self}` is not an iterator; try calling `.into_iter()` or `.iter()`"
-    ),
     label = "`{Self}` is not an iterator",
     message = "`{Self}` is not an iterator"
 )]