CamelSubscribable: Remove all asynchronous class methods.

CamelSubscribable: Remove all asynchronous class methods.

Rationale:

When I originally added Camel's asynchronous API, I thought providers
could choose to override the synchronous or asynchronous class methods.

Years later, turns out the asynchronous methods were never overridden,
and it wouldn't have worked anyway.  The asynchronous methods by default
invoke an associated synchronous function from a worker thread, but many
of Camel's synchronous functions do extra processing around invoking the
synchronous class method.  camel_store_get_folder_sync() is an example.

If a provider tried to implement synchronous methods in terms of the
asynchronous methods, then that extra processing in the synchronous
functions could potentially get skipped.

Removing the asynchronous class methods should help clarify how Camel
providers are intended to be written.

The removal should have no impact on existing Camel providers, all of
which implement only the synchronous class methods.  I've even padded
the CamelSubscribableInterface struct to keep the binary interface
intact.