Bug 691937 - Remove resources with e_source_remove_sync().

No doubt this could be made clearer in the API documentation, but a
collection backend should remove a server-side ESource the same way
as from a client process: e_source_remove().

EServerSideSource overrides ESource's remove() method.  Instead of
invoking a D-Bus method it removes itself and its children from the
registry server, then deletes the corresponding key files.

The EWS backend was calling e_source_registry_server_remove_source()
directly from its delete_resource_sync() method, so the key file was
never deleted.

(cherry picked from commit 79b5c0e7)