cpp-common/bt2: remove useless copy operations

cpp-common/bt2: remove useless copy operations

This patch removes useless copy constructors and assignment operators
from many wrapper classes in `cpp-common/bt2` where the compiler
generates some anyway.

Also, return by value (like everywhere else) from pseudo copy
operations.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I9e5b5e956905681c3a834122b3f0ed8311b5a6ae
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11302
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>