Remove enum_from_u32.

It's a macro that just creates an enum with a `from_u32` method. It has
two arms. One is unused and the other has a single use.

This commit inlines that single use and removes the whole macro. This
increases readability because we don't have two different macros
interacting (`enum_from_u32` and `language_item_table`).

compiler/rustc_data_structures/src/lib.rs

@@ -62,7 +62,6 @@ pub mod fx;
 pub mod graph;
 pub mod intern;
 pub mod jobserver;
-pub mod macros;
 pub mod marker;
 pub mod memmap;
 pub mod obligation_forest;