Upgrade to CiviCRM 4.3 failing on DROP FOREIGN KEY
This appears to be identical to CRM-12458.  Which is unusual - most cases where you'd get this error were solved in CRM-11260 but there are still edge cases that can cause this.

I would read those links in full - but my guess is you should edit CRM/Upgrade/Incremental/php/FourThree.php.  Remove the four highlighted lined at this link - that is, lines 883-886.

Since you're dropping a foreign key constraint that doesn't exist, there should be no difference as a result of removing these lines.  Note that you may have to remove some of the following lines if you get similar errors.