MDL-31414 mod_assign: Code cleanup for notification improvements
* No need to manually call update of providers that happens automatically.
* Added upgrade code for the new sendlatenotifications field.
* Added AMOS syntax for the two renamed strings
* Refactored assign::cron to use half as many DB queries
* Cleaned up unused vars and globals, coding style and phpdocs.
* Removed string notifications added previously but never used.
AMOS BEGIN
MOV [emailgradermail,mod_assign],[gradersubmissionupdatedtext,mod_assign]
MOV [emailgradermailhtml,mod_assign],[gradersubmissionupdatedhtml,mod_assign]
AMOS END
