Make iTunes Treat my iPhone Like A New Phone
Once you have selected that you want to encrypt the backup of an iPhone, it is not possible to uncheck without the password, or by erasing the content of the iPhone.
The information about the encryption status is saved on the iPhone, not in iTunes, so erasing all your backups wont do much in this situation.

This may seem weird to many people, but it's basically a safety measure.
Let's assume that someone broke in to your house and got away with both your computer and iPhone, got access to the computer because you had no Filevault enabled, and now wanted to move your data from your iPhone to your computer, and would be able to do so, just by erasing your old backups first. This would be somewhat of a security flaw in my opinion.

So to say it short, you need to erase your iPhone to disable the encrypted backup setting, if you do not know the password.