Use default_initializer in WorkerProcesses

This function was refactored out in Dask and still has the same
functionality. While `initialize_worker_process` does work, it does a
bit more than is needed (taking a UDF to initialize too). So just use
`default_initializer`.

Also there is no need to handle old Dask versions here since we now pin
Dask & Distributed to the exact same version. So drop the fallback code.

distributed/nanny.py

@@ -809,12 +809,9 @@ def _run(
         try:
             os.environ.update(env)
             dask.config.set(config)
-            try:
-                from dask.multiprocessing import initialize_worker_process
-            except ImportError:  # old Dask version
-                pass
-            else:
-                initialize_worker_process()
+
+            from dask.multiprocessing import default_initializer
+            default_initializer()
 
             if silence_logs:
                 logger.setLevel(silence_logs)