Replace T: 'a + Messenger with T: Messenger

reasons:

1. This annotation is no longer needed since it can be inferred from the fields present in the definitions in Rust 2018.
2. `T: 'a` annotation is never mentioned in previous chapters, this would make beinners confused.

references:

rust-lang/rfcs#2093
rust-lang/rust#44493
https://rust-lang-nursery.github.io/edition-guide/rust-2018/ownership-and-lifetimes/inference-in-structs.html

src/ch15-05-interior-mutability.md

@@ -140,7 +140,7 @@ pub trait Messenger {
     fn send(&self, msg: &str);
 }
 
-pub struct LimitTracker<'a, T: 'a + Messenger> {
+pub struct LimitTracker<'a, T: Messenger> {
     messenger: &'a T,
     value: usize,
     max: usize,