Remove TyCtxt::hir_krate.

It's a trivial wrapper around the `hir_crate` query with a small number
of uses.

compiler/rustc_driver_impl/src/pretty.rs

@@ -294,7 +294,7 @@ pub fn print<'tcx>(sess: &Session, ppm: PpMode, ex: PrintExtra<'tcx>) {
         }
         HirTree => {
             debug!("pretty printing HIR tree");
-            format!("{:#?}", ex.tcx().hir_krate())
+            format!("{:#?}", ex.tcx().hir_crate(()))
         }
         Mir => {
             let mut out = Vec::new();