flt.utils.muxer: subjective re-format

flt.utils.muxer: subjective re-format

Wrap arguments to `BT_CPPLOGE_APPEND_CAUSE_AND_THROW` so that we can
have slightly longer strings (still within 100 columns).  Remove
unnecessary local variables.

Change-Id: If7412e6913d06d12142f01e09742d4d6a5dc5d0e
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12505
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>