Abolish QueryVTable in favour of more assoc items on QueryConfig

This may introduce additional mono _but_ may help const fold things
better and especially may help not constructing a `QueryVTable` anymore
which is cheap but not free.

compiler/rustc_query_impl/src/lib.rs

@@ -34,7 +34,6 @@ use rustc_query_system::query::*;
 pub use rustc_query_system::query::{deadlock, QueryContext};
 
 pub use rustc_query_system::query::QueryConfig;
-pub(crate) use rustc_query_system::query::QueryVTable;
 
 mod on_disk_cache;
 pub use on_disk_cache::OnDiskCache;