MDL-40054 mod_lesson: replaced 'grade view' add_to_log call with an event
MDL-40054 mod_lesson: replaced 'update highscores' add_to_log call with an event
MDL-40054 mod_lesson: replaced 'view highscores' add_to_log call with an event
MDL-40054 mod_lesson: replaced 'start' add_to_log call with an event
MDL-40054 mod_lesson: replaced 'end' add_to_log call with an event
MDL-40054 mod_lesson: removed 'update email essay grade' add_to_log call

The add_to_log call was triggered whenever a grade was sent via the messaging
system. We do not need to fire this event here, it should be fired via the
messaging API.