Remove mail_folder_cache_service_added() and rely instead on
mail_folder_cache_service_enabled()/disabled(). Despite the function
names the folder cache doesn't actually care about enabled status, it
simply maintains a set of CamelStores to monitor. We should refactor
the folder cache API to better reflect that.
EMailAccountStore always immediately follows a "service-added" signal
emission with either a "service-enabled" or "service-disabled" emission,
which will add or remove the service from the folder cache.
a36294e7c7