bt2: remove _GenericMessageIterator abstraction level

bt2: remove _GenericMessageIterator abstraction level

We used to have two message iterator types sharing a lot,
_OutputPortMessageIterator and _UserComponentInputPortMessageIterator,
so they shared some code in the name of _GenericMessageIterator.
_OutputPortMessageIterator is no more, so the _GenericMessageIterator
abstraction is not useful anymore.  Rename to
_UserComponentInputPortMessageIterator and remove all the templating.

Change-Id: If626aeac881ab2ca478a196a53047b4bcf3b7a65
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2018
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>