Use rustc_fluent_macro::fluent_messages! directly.

Currently we always do this:
```
use rustc_fluent_macro::fluent_messages;
...
fluent_messages! { "./example.ftl" }
```
But there is no need, we can just do this everywhere:
```
rustc_fluent_macro::fluent_messages! { "./example.ftl" }
```
which is shorter.

compiler/rustc_interface/src/lib.rs

@@ -13,8 +13,6 @@
 #[macro_use]
 extern crate tracing;
 
-use rustc_fluent_macro::fluent_messages;
-
 mod callbacks;
 mod errors;
 pub mod interface;
@@ -31,4 +29,4 @@ pub use queries::Queries;
 #[cfg(test)]
 mod tests;
 
-fluent_messages! { "../messages.ftl" }
+rustc_fluent_macro::fluent_messages! { "../messages.ftl" }