You cannot make a backup without that password. That is precisely what the system of encrypted backups is designed to prevent. If it were possible to bypass the encryption setting and make an unencrypted backup of your phone, then someone could gain access to that information by simply backing up your device, rendering the very point of encrypting your information moot.
To remove the requirement for the encryption password, you will have to erase your device and set it up as new. This will delete all information from the device, which is by design in the event someone does not have (I.e. you, in this case) the password.
There is no alternate solution. If you forget the password, you will HAVE to erase the device and set it up as a new device. That is your only means of removing the phone's setting that now requires the encryption password to make a backup in iTunes.