Remove a fallacy in scheduler.driver config option help text

Change fe3d6dba3d removed the
ability to specify a full classpath to load up your own
scheduler driver. After that point stevedore extensions had
to be used from setup.cfg.

Change 6832a417c9 encoded the
supported in-tree scheduler driver options by using the
"choices" kwarg with the scheduler.driver option. This means
if you specify something not in that list, you get a ValueError.

So the part in the help text about registering your own
stevedore extension is a lie since it would still fail because
of "choices".

Change-Id: I60ce26f7370b71b2dc266d582d9b17d35e8cd9c1

nova/conf/scheduler.py

@@ -76,8 +76,6 @@ Possible values:
    multiple schedulers
 ** 'chance_scheduler', which simply picks a host at random
 ** 'fake_scheduler', which is used for testing
-** A custom scheduler driver. In this case, you will be responsible for
-   creating and maintaining the entry point in your 'setup.cfg' file
 """),
     cfg.IntOpt("periodic_task_interval",
         default=60,