Remove the opportunity to configure script_dir for LocalChannel

Prior to this PR, if the argument defaulted to None, then the DFK would
set a script dir based on the workflow rundir.

Post this PR, this default behaviour is the only behaviour and the user
cannot specify an overriding value.

Future PRs will remove channel level script directories entirely.
This is part of issues #3515 removing channels,

parsl/channels/local/local.py

@@ -15,14 +15,14 @@ class LocalChannel(Channel, RepresentationMixin):
     and done so infrequently that they do not need a persistent channel
     '''
 
-    def __init__(self, script_dir=None):
+    def __init__(self):
         ''' Initialize the local channel. script_dir is required by set to a default.
 
         KwArgs:
             - script_dir (string): Directory to place scripts
         '''
         self.hostname = "localhost"
-        self.script_dir = script_dir
+        self.script_dir = None
 
     def execute_wait(self, cmd, walltime=None):
         ''' Synchronously execute a commandline string on the shell.