Remove exported/unexported ordering override

[laborg]

Three reasons I think this should be removed:

1) The documentation for @index states:

Order and Modules behave the same way as in @autodocs blocks and filter out docstrings that do not match one of the modules or categories specified.

which is not true due to the split introduced into exported/unexported bindings only in @autodocs.

2) The documentation contains an example which shows how to use the Private and Public field with @autodocs to create the currently implemented behaviour.

3) Offering the possibility to define an Order, but overridden it by something that is not documented is quite surprising and results in bug reports.

Fixes: #964

[mortenpi]

Sorry for not reviewing earlier, but LGTM. I agree that we shouldn't be overriding the other ordering rules with an undocumented rule. It actually makes it impossible to properly sort your docstrings with at-autodocs in the case when you don't want to distinguish between exported and unexported names. And, as you say, if you do want to sort them by their export status, you can use two consecutive at-autodocs blocks.