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_mir_build/src/lib.rs

@@ -24,9 +24,7 @@ mod thir;
 
 use rustc_middle::query::Providers;
 
-use rustc_fluent_macro::fluent_messages;
-
-fluent_messages! { "../messages.ftl" }
+rustc_fluent_macro::fluent_messages! { "../messages.ftl" }
 
 pub fn provide(providers: &mut Providers) {
     providers.check_match = thir::pattern::check_match;