Revert "[VM] Fix for issue 34839 - Dart_Cleanup may hang while shutti…

…ng down"

This reverts commit 89c845e.

Reason for revert: Seems to cause ASAN failures

Original change's description:
> [VM] Fix for issue 34839 - Dart_Cleanup may hang while shutting down
>      the service isolate.
> 
>      The service isolate request for shutdown uses a Dart level message
>      for exiting the isolate, this shuts down the isolate but does not
>      completely bring the isolate down if there are some open ports. We
>      have a timer isolate started in the VM isolate and if the shutdown
>      happens soon enough this port is still open.
> 
> Change-Id: Icfa07c91b8692eb76ba8502063cf6f5cf04832a5
> Reviewed-on: https://dart-review.googlesource.com/c/81200
> Reviewed-by: Ryan Macnak <rmacnak@google.com>
> Commit-Queue: Siva Annamalai <asiva@google.com>

TBR=rmacnak@google.com,asiva@google.com

Change-Id: I0fd4472d277a380b739e602aa3910cc1f96ed89e
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/81223
Reviewed-by: Siva Annamalai <asiva@google.com>
Commit-Queue: Siva Annamalai <asiva@google.com>

runtime/lib/vmservice.cc

@@ -140,7 +140,6 @@ DEFINE_NATIVE_ENTRY(VMService_OnExit, 0) {
     OS::PrintErr("vm-service: live ports = %" Pd "\n",
                  message_handler->live_ports());
   }
-  Isolate::KillIfExists(isolate, Isolate::kInternalKillMsg);
   return Object::null();
 }