remove obsolete idle statement from Unsafe.getLongVolatile

Now that Josh has fixed the busy block bug (see commit 10d15d2), we
don't need this anymore.

src/builtin.cpp

@@ -765,10 +765,6 @@ Avian_sun_misc_Unsafe_getLongVolatile
   object o = reinterpret_cast<object>(arguments[1]);
   int64_t offset; memcpy(&offset, arguments + 2, 8);
 
-  // avoid blocking the VM if this is being called in a busy loop
-  PROTECT(t, o);
-  { ENTER(t, Thread::IdleState); }
-
   object field;
   if (BytesPerWord < 8) {
     field = fieldForOffset(t, o, offset);