The user activity logging is stored in the o_loggin table. For historic reason user properties and the identity name (username) is also added to each row for later data mining.
This is a flawed design in many ways. There is no need to have this data in the first place since it can easily be generated when exporting the data periodically to an external data mining database by joining the appropriate tables. It also makes anonymising of the data necessary when a user is deleted. The data in the logging table must be kept when a user is deleted because it contains important information about the usage of learning resources from which statistics are generated and it is also used to create an audit log for administrative actions within courses.
With the dropping of the unnecessary tables the logging data is already anonymised. When a user is deleted, all his user properties are deleted as well and thus it is not possible to reveal the users identity by joining the data with the identity or user table.
Since the data is used to generate the course log files that can be downloaded by the course owner, the course log must be adapted to get the user data by joining with the database during log generation.