Remove prof_threshold built-in event

[spredolac]

Remove prof_threshold built-in event. It is simple to implement it as user event when needed. For example,

   void onMemoryThresholdHook(bool /*is_alloc*/, uint64_t alloc, uint64_t dealloc) {
        ...
   }
    
   static struct {
     void (*callback)(bool, uint64_t, uint64_t);
     uint64_t interval;
     bool is_alloc_only;
   } uobj = {
       .callback = &onMemoryThresholdHook,
       .interval = 128 * 1024ULL * 1024ULL,
       .is_alloc_only = true, // Only fire for allocation events
   };
   int err = mallctl("experimental.hooks.thread_event", NULL, NULL, &uobj, sizeof(uobj));
   ```
   
   so there is no need to keep this one in.