Revert "Updates problem-builder version to use 2.6.0"

https://openedx.atlassian.net/browse/TNL-5903

This reverts commit d6787cd1.
This reverts commit 0016d1af.

requirements/edx/edx-private.txt

@@ -8,7 +8,7 @@
 
 # For Harvard courses:
 -e git+https://github.com/gsehub/xblock-mentoring.git@4d1cce78dc232d5da6ffd73817b5c490e87a6eee#egg=xblock-mentoring
-git+https://github.com/open-craft/problem-builder.git@v2.6.0#egg=xblock-problem-builder==2.6.0
+git+https://github.com/open-craft/problem-builder.git@v2.0.4#egg=xblock-problem-builder==2.0.4
 
 # Oppia XBlock
 -e git+https://github.com/oppia/xblock.git@9f6b95b7eb7dbabb96b77198a3202604f96adf65#egg=oppia-xblock