Remove unused re-used DFK default configuration object (#2864)

The default configuration actually comes from DataFlowKernelLoader.load()
and the Config() object specified in the DataFlowKernel.__init__ default
parameters is never used, as all constructions of DataFlowKernel in the
codebase specify a config.

This comes from flake8-bugbear

parsl/dataflow/dflow.py

@@ -69,7 +69,7 @@ class DataFlowKernel:
     """
 
     @typechecked
-    def __init__(self, config: Config = Config()) -> None:
+    def __init__(self, config: Config) -> None:
         """Initialize the DataFlowKernel.
 
         Parameters