cpp-common: remove unused include

cpp-common: remove unused include

Remove some unused (according to clangd) includes.  Add one IWYU pragma
to tell it to keep `fmt/format.h`, which is needed to make the
`format_as` function defined in that file work.

Change-Id: I9c4d716975e46bbfad0372aa7ede5f449868848f
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11782
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>