src.ctf.lttng-live: remove unused variable in live_get_msg_ts_ns

src.ctf.lttng-live: remove unused variable in live_get_msg_ts_ns

clock_class is assigned, but never used outside of assertions, remove
it (I don't think the assertions are particularly important here).

Change-Id: I8b7e23bb14d51837d6120567cb303a0436c8648a
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8129
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>