8263030: Remove Shenandoah leftovers from ReferenceProcessor

Backport-of: ef5e13d263b6d4aaa50e777f7a3fc818293126c8

src/hotspot/share/gc/shared/referenceProcessor.cpp

@@ -1164,8 +1164,7 @@ bool ReferenceProcessor::discover_reference(oop obj, ReferenceType rt) {
       // Check assumption that an object is not potentially
       // discovered twice except by concurrent collectors that potentially
       // trace the same Reference object twice.
-      assert(UseG1GC || UseShenandoahGC,
-             "Only possible with a concurrent marking collector");
+      assert(UseG1GC, "Only possible with a concurrent marking collector");
       return true;
     }
   }