Merge QueryDescription into QueryConfig

`QueryDescription` has gone through a lot of refactoring and doesn't
make sense anymore.

compiler/rustc_query_impl/src/lib.rs

@@ -36,7 +36,7 @@ mod keys;
 use keys::Key;
 
 pub use rustc_query_system::query::QueryConfig;
-pub(crate) use rustc_query_system::query::{QueryDescription, QueryVTable};
+pub(crate) use rustc_query_system::query::QueryVTable;
 
 mod on_disk_cache;
 pub use on_disk_cache::OnDiskCache;