Unused code in check_nest_attributes function

From a -Wunused build:

`src/hotspot/share/prims/jvmtiRedefineClasses.cpp:702:21: warning: unused variable 'state' [-Wunused-variable]` (edited)
  `JvmtiThreadState *state = JvmtiThreadState::state_for((JavaThread*)thread);`


static jvmtiError check_nest_attributes(InstanceKlass* the_class,
                                        InstanceKlass* scratch_class) {
  // Check whether the class NestHost attribute has been changed.
  Thread* thread = Thread::current();
  ResourceMark rm(thread);
  JvmtiThreadState *state = JvmtiThreadState::state_for((JavaThread*)thread);

The state variable was leftover from an earlier version of the code that used a RedefineVerifyMark. It is not needed and can be deleted.