auto_update_weights calls update() even if there is no aggregationcoef2 change

As most of the gradebook performance reports are related with full gradebook regrades, I've created a test course (tool_generator - M size) with around 1000 users and 100 activities to see the basic configuration issues. I've been changing the gradebook structure and checking which interactions with the database we have to see if there is anything we could improve.

New grade category and manual grade items results in around 1000-1500 db reads and 400-700 db writes, I see many stuff and I don't want to mess up with the whole gradebook design nor implement major changes here, but auto_update_weights seems to be calling update() unnecessarily. I've reduced the number of db writes to half of it with the provided patch. I haven't seen any gradeitem modifications to justify an update() call if the aggregatecoef2 value is not changed. As a beneficial side effect this reduces the number of records added to grade history which is also the reason why we have that many insert record calls.