tests: remove unnecessary message iterator classes

These user message iterator classes don't do anything special, we can
remove them and pass bt2._UserMessageIterator directly to the components
instead, reducing a little bit the noise in the tests.

Some classes in test_component_class.py were simply unused, so I removed
them.

Change-Id: I79f787b9d121a3dc6456f81090ebf51d088d2c73
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2736
Tested-by: jenkins <jenkins@lttng.org>