performance improvement for get_users_in_context (take #2)

This is a followup of MDL-63785 where a good number of privacy queries were simplified to avoid joining with the contexts table when the contexts being searched are block, course or user. Simply because the instanceid of those contexts that we are fetching now is, already, the block, course or user id that we have in the different tables.

This issue is about to look for remaining cases (some were detected in the original issue, there may be more) to follow the same simplification approach if the original one is accepted.

Ciao