Remove some unused includes in C++ files

Remove some unused includes in C++ files

Remove some unused include files.  This was done partly looking at
"unused-includes" warnings from clangd in my IDE, and partly with
include-what-you-use.

Whenever needed, add some includes to keep things compiling (where
some header files relied on previous inclusions).

Change-Id: I0bca7aba4c172a13474a76b08e4a0b543e152733
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11493
Tested-by: jenkins <jenkins@lttng.org>