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_ast_passes/src/lib.rs

@@ -15,12 +15,10 @@
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]
 
-use rustc_fluent_macro::fluent_messages;
-
 pub mod ast_validation;
 mod errors;
 pub mod feature_gate;
 pub mod node_count;
 pub mod show_span;
 
-fluent_messages! { "../messages.ftl" }
+rustc_fluent_macro::fluent_messages! { "../messages.ftl" }