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_const_eval/src/lib.rs

@@ -39,10 +39,9 @@ pub mod util;
 
 pub use errors::ReportErrorExt;
 
-use rustc_fluent_macro::fluent_messages;
 use rustc_middle::{ty, util::Providers};
 
-fluent_messages! { "../messages.ftl" }
+rustc_fluent_macro::fluent_messages! { "../messages.ftl" }
 
 pub fn provide(providers: &mut Providers) {
     const_eval::provide(providers);