Cleanup task for content bank, to remove H5P contents without a valid file

An issue was raised in MoodleCloud related to the content bank: MC-4062

In some cases, loading the content bank, from a course page, gives an "Invalid H5P content URL", not offering any way to delete the offending content or create a new one. A fix was integrated in MDL-72358 (to avoid displaying this error). However, we should try to figure out why it happened and add some cleanup task to remove the DB records that will remain forever in DB (and nobody will see them ever).