MDL-36941 core: convert existing api to use new table structure

Also deprecated the following functions -

1. message_move_userfrom_unread2read - It is not necessary
   for us to mark a message as read on user deletion.
2. message_get_blocked_users - Horrible logic used to
   determine if a user is blocked via reference on some
   randomly chosen 'isblocked' variable.
3. message_get_contacts - The same as above. This can be
   done in a much nicer way.
4. message_mark_message_read - We want two functions to do
   this to avoid confusing messages and notifications.
5. message_can_delete_message - This assumed the variable
   $message contained the 'useridto' property, which
   was present in the old table structure. We do not want
   future usages where a query is done on the new table
   and is simply passed as this won't contain this property.
6. message_delete_message - Same as above.

lib/moodlelib.php

@@ -4071,9 +4071,6 @@ function delete_user(stdClass $user) {
     // Delete all grades - backup is kept in grade_grades_history table.
     grade_user_delete($user->id);
 
-    // Move unread messages from this user to read.
-    message_move_userfrom_unread2read($user->id);
-
     // TODO: remove from cohorts using standard API here.
 
     // Remove user tags.