Remove changegradewarning string and init_grade_change functions from mod_assign

This issue only removes leftover code, which is not being callled anywhere (provided MDL-53392 is integrated first!). To be thorough however, please follow the testing instructions from MDL-53392 , which I've included (changed to suit master only) below: Prerequisites: You must have a course with a student enrolled. Test Steps: As an admin user: Navigate to your course home page Turn editing on Create a new assignment with default settings Assign a grade to the enrolled student for the assignment you just created Disable javascript Navigate back to course home and edit your assignment settings Scroll to the 'Grade' section and confirm that : a) The grade field is accompanied by the message - " Some grades have already been awarded, so the grade type cannot be changed. If you wish to change the maximum grade, you must first choose whether or not to rescale existing grades. " b) There are no other messages/text warnings presented in the Grade section. c) The 'Maximum Grade' field IS editable. Change the maximum grade to 10000 but leave the 'Rescale existing grades' select set to 'Choose'. Save the form Confirm that the form fails validation Change the maximum grade to 50 and change the 'Rescale existing grades' select set to 'Yes'. Save the form Confirm that the form saves successfully Enable javascript Navigate back to course home and edit your assignment settings Scroll to the 'Grade' section and confirm that : a) The grade field is accompanied by the message - " Some grades have already been awarded, so the grade type cannot be changed. If you wish to change the maximum grade, you must first choose whether or not to rescale existing grades. " b) There are no other messages/text warnings presented in the Grade section. c) The 'Maximum Grade' field IS NOT editable. Change the 'Maximum grade' to 80 and change the 'Rescale existing grades' select set to 'Yes'. Save the form Confirm that the form saves successfully