remove IntoIterator impl for &Binders

[basil-cow]

Removing a redundant IntoIterator impl for &Binders following this zulip convo.

The reason to do this is
a) you can achieve the same effect by doing binders.as_ref().into_iter()
b) because of the somewhat recursive nature of the definition (both Item and Self contain Binders<_>) it often results in cryptic errors if typechecking is unsuccessfull (you can check zulip convo for examples).