bt2: remove probing of BABELTRACE_PYTHON_BT2_NO_TRACEBACK env var

bt2: remove probing of BABELTRACE_PYTHON_BT2_NO_TRACEBACK env var

This was used previously when handling exceptions thrown by user code,
to decide whether we printed exceptions or not.  Nowadays, we let
exceptions propagate to the native code, which logs it.  I believe we
can remove this.

Change-Id: I14adae265c83a169d176dda9d83d4d05fcd26edc
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1636
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>