Remove 'Select' option from price option visibility drop-down (undefined index visibility_id error)

Since CRM-12167 we can make price options 'Public' or 'Admin.

However, this 'visibility_id' field is not mandatory when creating the options. When it is not set, you receive an 'undefined index' error on the front end registration form.

Notice: Undefined index: visibility_id in CRM_Event_Form_Registration_Register::buildAmount() (line 598 of /srv/buildkit/build/dmaster/sites/all/modules/civicrm/CRM/Event/Form/Registration/Register.php).

Steps to recreate:

• Create a priceset
• Create a price field, type 'Select'
• When adding price options, notice that it is possible to change the 'Visibility' to '- select -'
• Do that, and save
• Add the price set to an event
• View the event registration form as an anonymous user
• Notice the error message

Mitigation:

• The form does default to 'public', so it's unlikely anyone would change this to '- select -'

Steps to fix:

• Remove the '- select -' option from the visibility drop down on price options.