python: remove "all" filter name in add_filter()

Problem: The JobList class's add_filter() function checks for the
special filter name "all" and sets the filter to pending and running.
As far as I can tell, the special filter name "all" has never been
supported or documented.  In addition, it doesn't make much sense
since it should set pending, running, and inactive to get all jobs.
My suspicion is this may be left over code from development that
did not get removed.

Remove check for the filter name "all".

src/bindings/python/flux/job/list.py

@@ -236,11 +236,6 @@ def set_user(self, user):
     def add_filter(self, fname):
         """Append a state or result filter to JobList query"""
         fname = fname.lower()
-        if fname == "all":
-            self.states |= self.STATES["pending"]
-            self.states |= self.STATES["running"]
-            return
-
         if fname in self.STATES:
             self.states |= self.STATES[fname]
         elif fname in self.RESULTS: