Remove the add_option() fallback

Previously, we were attempting to explicitly add the option first and only
calling `update_option()` the option didn't exist. This was both to
explicitly force the option to autoload and also to ensure a minimum
amount of code was called (`update_option()` runs several filters and
eventually defers to `add_option()` for a new option anyway). However, to
avoid confusion, we'll just settle with `update_option()` regardless if
the option exists or not.