Remove the example of the empty import shell customization

[Salvo Polizzi]

The final section of this ​tutorial currently states:

If you prefer to not have models automatically imported, create a custom get_namespace() that excludes the super().get_namespace() call:

from django.core.management.commands import shell

class Command(shell.Command):
    def get_namespace(self):
        return {}

---

This section can be replaced with a simpler explanation:

To disable automatic importing entirely, use the --no-imports flag.

[Sarah Boyce]

I think you mean ​this section and I think it makes sense to remove the part "If you prefer to not have models automatically..." and example entirely as the --no-imports flag is already documented 👍