Remove unused `tests/plugins/test_utils_muxer_complete.in`

This script was the runner for the `flt.utils.muxer` test running in C,
which does not exist anymore. We'll write `flt.utils.muxer` tests either
in Python or with `sink.text.details` and expectation files.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I28df6d20c3bd3ee0b056790046d041787e702430
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1422
Tested-by: jenkins
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>