flt.utils.muxer: don't reset clock expectation when seeking beginning

When 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.

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Change-Id: I2ea587ea7b7f62ae509632836ae4364292903001
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12148
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>