AsyncLogWriter should not overide is_Named_thread()

Comments from David Holmes:
"
BTW: I noticed a separate bug in the crash log. You've defined:

bool is_Named_thread() const override { return true; }

for the AsyncLogWriter, but as you pointed out when I asked, this thread
can't be a NamedThread because that is a special kind of GC thread. So
that method should not be there.
"

I misunderstood is_Named_thread(). I thought it should return true as long as it has a name. It's incorrect! Only NamedThread and its subclasses return true.

Code like this is undefined for AsyncLogWriter. Its vtable doesn't have the entry for gc_id().

uint GCId::current_or_undefined() {
  return Thread::current()->is_Named_thread() ? currentNamedthread()->gc_id() : undefined();
}