Revert 'Stabilize -Znext-solver=coherence'

compiler/rustc_trait_selection/src/traits/engine.rs

@@ -35,8 +35,10 @@ where
         if infcx.next_trait_solver() {
             Box::new(NextFulfillmentCtxt::new(infcx))
         } else {
+            let new_solver_globally =
+                infcx.tcx.sess.opts.unstable_opts.next_solver.map_or(false, |c| c.globally);
             assert!(
-                !infcx.tcx.next_trait_solver_globally(),
+                !new_solver_globally,
                 "using old solver even though new solver is enabled globally"
             );
             Box::new(FulfillmentContext::new(infcx))