Make Devise find the strategy class automatically

Since we're already setting `wordpress_oauth2` using the `option :name`
command in the `OmniAuth::Strategies::Wordpress` class, Devise can
automatically find the strategy. However, it wasn't working because we
were passing a string instead of a symbol.

config/initializers/devise.rb

@@ -284,7 +284,6 @@ Devise.setup do |config|
   config.omniauth :wordpress_oauth2,
                   Rails.application.secrets.wordpress_oauth2_key,
                   Rails.application.secrets.wordpress_oauth2_secret,
-                  strategy_class: OmniAuth::Strategies::Wordpress,
                   client_options: { site: Rails.application.secrets.wordpress_oauth2_site },
                   setup: OmniauthTenantSetup.wordpress_oauth2