fix: remove caching for lti_access_to_learners_editable

This commit removes the request_cached decorator on the method lti_access_to_learners_editable. The caching was not working correctly. It appeared that the cached value was not being recomputed on a per-request basis, which meant that the cached value was not successfully being updated. Also, the initial cached value was computed incorrectly. The effect was that the PII sharing XBlock fields were always being displayed in Studio and CourseAllowPIISharingInLTIFlag instances were not being created for courses that had PII sharing XBlocks that did not already have CourseAllowPIISharingInLTIFlag instances.

Note that this commit fixes the backwards compatibility of the CourseAllowPIISharingInLTIFlag. This means that CourseAllowPIISharingInLTIFlag will now be created for all courses that contain an LTI XBlock that shares username and/or email for which a CourseAllowPIISharingInLTIFlag does not already exist.

Time was spent on determining why the caching was failing, but it was challenging. If the issue is in the request_cached decorator, then the impact of changing it is large - it's used elsewhere in the platform. The need for a fix was urgent, and the supposed performance enhancement of leaving the caching in doesn't justify delaying a fix.