Remove lttng_ust_synchronize_trace public symbol

Remove 'lttng_ust_synchronize_trace', an ABI symbol of the
liblttng-ust-tracepoint library that only wraps
'lttng_ust_urcu_synchronize_rcu', a common extern symbol across
all libraries.

Replace it with direct calls to 'lttng_ust_urcu_synchronize_rcu'.

Change-Id: Ieed2e4a195bb1cfab5a97d746024a6a1c5854362
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>