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_expand/src/lib.rs

@@ -23,8 +23,6 @@ extern crate tracing;
 
 extern crate proc_macro as pm;
 
-use rustc_fluent_macro::fluent_messages;
-
 mod placeholders;
 mod proc_macro_server;
 
@@ -66,4 +64,4 @@ mod mut_visit {
     mod tests;
 }
 
-fluent_messages! { "../messages.ftl" }
+rustc_fluent_macro::fluent_messages! { "../messages.ftl" }