edition lint: declarations obviated by in-band lifetimes

[nrc]

cc #44524

e.g., in fn two_args<'b>(arg1: &Foo, arg2: &'b Bar) -> &'b Baz, suggest that <'b> is not necessary.

[nikomatsakis]

We decided that in-band lifetimes are not going to be stabilized for Rust 2018. Therefore, I nominate this for removal from the milestone.

[steveklabnik]

Triage: no movement here