Remove the "webform_debug" variable

Issue Summary

The "webform_debug" variables allows you to print out debugging information after creating submissions. This was intended as a development tool mostly for viewing the contents of e-mails without actually sending them. However I personally don't use it since I have a local mail server set up, and alternatively you can now log e-mails to watchdog instead of sending them using the devel module. And of course if you're doing development, you could always just put these debugging statements directly in the code if you needed them. For these reasons, I think it's time to say good-bye to this variable, which should prevent unnecessary confusion when this option gets accidentally enabled.