storage: remove broken default parameter from isodate()

With that syntax, the default timestamp would have been from the time of
the function's definition (not invocation). But all callers are passing
an explicit timestamp anyway.

qubes/storage/__init__.py

@@ -874,7 +874,7 @@ def driver_parameters(name):
     return [p for p in params if p not in ignored_params]
 
 
-def isodate(seconds=time.time()):
+def isodate(seconds):
     ''' Helper method which returns an iso date '''
     return datetime.utcfromtimestamp(seconds).isoformat("T")