[dagster-university] removes DU content as it has been migrated to project-dagster-university (dagster-io#27621)

## Summary & Motivation

dagster-io/project-dagster-university#53

## How I Tested These Changes

## Changelog

NOCHANGELOG

Author: cmpadden

.github/workflows/build-dagster-university.yml

@@ -217,7 +217,6 @@ Each MDX file in `/content` directly corresponds to a single, rendered docs page
 - [**Code snippets**](#code-snippets) are embedded throughout the prose docs. All code snippets used in the docs site live (outside the `docs` folder) in `examples/docs_snippets`. This is just a regular python package, which makes it easy to test and specify dependencies for the snippets.
 - [**Screenshots**](#screenshots) are image files living in `next/public/images`, typically under a subdirectory corresponding to the page on which they appear. There's no need to manually manage the screenshots in this directory-- instead you can add a specification your screenshot to `screenshots` and run `dagster-ui-screenshot capture` to quasi-automatically generate the screenshot and place it in the appropriate subdirectory of `next/public/images`.
 - [**Navigation schema**](#navigation-schema) is a JSON file specifying the contents of the main nav for the site (found in the left sidebar). This typically needs to be updated to include a link when new prose doc is added.
-- [**Dagster University**](#dagster-university) is a separate Nextjs site containing the content for Dagster University courses. 
 
 Refer to the [Development section][docs-development] for info about committing and pushing MDX changes.
 
@@ -852,12 +851,6 @@ Quote
 
 ---
 
-## Dagster University
-
-Refer to the Dagster University [README](https://github.com/dagster-io/dagster/tree/master/docs/dagster-university) for more info about working in this directory.
-
----
-
 ## Troubleshooting
 
 - [Error: ModuleNotFoundError: No module named 'X']
@@ -969,4 +962,4 @@ To resolve the error, you need to rebuild your `tox` environment and bring in th
 <!-- ## REFERENCES -->
 
 [reference-cli-commands]: #cli-commands
-[reference-markdown-rst]: #markdown-and-restructuredtext-formatting
+[reference-markdown-rst]: #markdown-and-restructuredtext-formatting