Remove unnecessary inclusions of "internal public" headers

Remove unnecessary inclusions of "internal public" headers

I believe that these spots don't really need to include files from
babeltrace2 other than babeltrace.h.  In fact, given the presence of:

    #ifndef __BT_IN_BABELTRACE_H
    # error "Please include <babeltrace2/babeltrace.h> instead."
    #endif

in those "internal public" header files and the fact that the including
files don't define __BT_IN_BABELTRACE_H, I believe that those inclusions
don't have an effect.  It must be that the included file was already
included earlier, as part of babeltrace.h.

Change-Id: I9f88123f12f916dea8c90641dd7b32c531f5c707
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11458
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>