mentoring-noworkbench: Remove import of workbench module (moved to SDK)

@nedbat @wedaly Patch for the import error with the latest XBlock repository. Let me know if that fixes it for you.

Relevant commit: https://github.com/gsehub/xblock-mentoring/commit/33ec4a0dfc5034c3499c8d50b197f85ca8625231

TKTS-1238