BACKUP ENCRYPTION

You don't remove it.

If you encrypted your backup and now can't remember the password, the backup is gone and can't be used.

When you turned on encryption you were warned about this.

About encrypted backups on your iPhone, iPad, or iPod touch - Apple Support

Backup passwords cannot be created on their own. As a matter of fact, it is a two step process. You have to enter the password once, then you have to verify it before it takes. You had to do it sometime.