IOTBTOOL-333: Fix SimpleQueue build failure on py3

Build system was using an internal feature of the Pool class that is unavailable
in a py3 system. This would cause an exception if tool execution failed.
Offending code has now been removed

Author: mark-edgeworth

tools/toolchains/mbed_toolchain.py

@@ -556,9 +556,7 @@ def compile_queue(self, queue, objects):
                             ])
                         objects.append(result['object'])
                     except ToolException as err:
-                        if p._taskqueue.queue:
-                            p._taskqueue.queue.clear()
-                            sleep(0.5)
+                        # Stop the worker processes immediately without completing outstanding work
                         p.terminate()
                         p.join()
                         raise ToolException(err)