CRM-14522 remove incorrect FK

@totten  - don't want to disturb you right now just for this - but what is best way to remove FK_civicrm_msg_template_pdf_format_id from civicrm_message_template in the upgrade script (feel free just to add as a commit :-))

I feel like there is special code around FKS. BTW - you DID introduce this bug - so you should help fix - it was in that really buggy commit you did with the message 'Import from svn'