bt2::internal::BorrowedObj: use default copy operations explicitly

bt2::internal::BorrowedObj: use default copy operations explicitly

What was called the generic copy constructor and assignment operator
are in fact constructor templates, not true copy operations.

This is because C++ requires that a copy constructor/assignment operator
be a "non-template non-static member function".

This patch is not a fix because the generated default copy operations
were fine (just copy the underlying libbabeltrace2 pointer). Just use
the default ones explicitly and fix the comments so that we know what's
going on.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I4b4f19416a78ca05eaf3fa92f85f9637b2be2c0a
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8090
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10801
Tested-by: jenkins <jenkins@lttng.org>