tests: fix all basic type check issues of lttng_live_server.py

Do the necessary changes such that this comment passes cleanly:

    $ PYTHONPATH=/home/smarchi/src/babeltrace/tests/utils/python pyright tests/data/plugins/src.ctf.lttng-live/lttng_live_server.py

Pyright pointed out that field _trace_session_state didn't exist, in:

    @property
    def trace_session_state(self):
        return self._trace_session_state

The actual field name is _ts_state.  However, it shows that this
property is never used, so remove it.

Change-Id: I115fe473310ecdb1e2fd2e5d369aae33afe963f1
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10893
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>