[DEPR] Clean up references to Askbot

[dianakhuang]

All of the code for this is gone, so we just want to clean up references that no longer make any sense.

Here's the search results: https://github.com/search?q=%28org%3Aopenedx%29+askbot&type=code

We want to leave the references in the edx-documentation repository alone, but everything else can be deleted.

[Yagnesh1998]

assign me