Implement Disable Provider method

[adamwoodnz]

Two_Factor_Core provides a method enable_provider_for_user.

To compliment this, a method should be added disable_provider_for_user, which takes similar arguments ( $user_id, $provider ) and also returns a boolean.

[dd32]

@adamwoodnz Can you update this with a two-factor specific request, rather than the wporg-two-factor requirements?

I think this is something like "There should be an API to compliment enable_provider_for_user() to disable a provider"?