Reduce the number of Makefiles in 'src/plugins/utils'

Reduce the number of Makefiles in 'src/plugins/utils'

Continue the effort started by Simon to reduce the overall number of
Makefiles to allow for faster parallel builds. Also reduce the number of
convenience libraries for faster linking.

Change-Id: I772572c3dc1d2481ec94526a4301d4bb79f9d855
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11151
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>