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_incremental/src/lib.rs

@@ -28,6 +28,4 @@ pub use persist::save_work_product_index;
 pub use persist::setup_dep_graph;
 pub use persist::LoadResult;
 
-use rustc_fluent_macro::fluent_messages;
-
-fluent_messages! { "../messages.ftl" }
+rustc_fluent_macro::fluent_messages! { "../messages.ftl" }