lib/graph/iterator: don't reset clock expectation when seeking beginning

When asking an iterator to seeking the beginning, there is no reason to
reset the clock expectations.  The clocks seen after a seek beginning
should be compatible with those seen before.

Change-Id: I91d45b22c477048d92bc567a3ee26ec9641c1055
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12147
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>