As the connection is a singleton, it does not make sense to have a
construction flag. Calling tracker_sparql_connection_get and
tracker_sparql_connection_get_direct in the same process triggered an
assertion failure. This changes tracker_sparql_connection_get_direct to
be equivalent to tracker_sparql_connection_get.

The D-Bus proxies in libtracker-bus use lazy initialization, so they do
not cause any overhead when constructing the connection.

Direct-only can still be enforced on the process level by setting
TRACKER_SPARQL_BACKEND=direct.