Remove extraneous changes beyond avoiding the false positive warnings, since super.shutdown() is never expected to throw.

Also remove redundant unit test.

Rework of PR grpc#12705.

Author: kannanjgithub

core/src/main/java/io/grpc/internal/ManagedChannelOrphanWrapper.java

@@ -72,13 +72,12 @@ public ManagedChannel shutdown() {
 
   @Override
   public ManagedChannel shutdownNow() {
-    ManagedChannel result = super.shutdownNow();
     phantom.clearSafely();
     // This dummy check prevents the JIT from collecting 'this' too early
     if (this.getClass() == null) {
       throw new AssertionError();
     }
-    return result;
+    return super.shutdownNow();
   }
 
   @VisibleForTesting