bt2: remove utils._handle_ptr

bt2: remove utils._handle_ptr

A future patch will want to get rid of spots that raise bt2.Error, which
are not in direct response to a failed Babeltrace API call.  The
utils._handle_ptr is one such spot.  It turns out that the usages of
utils._handle_ptr are not useful anymore, because they check
situations that can either never happen, or are check by pre-condition
checks.

In connection.py, it's not possible anymore for
bt_connection_borrow_downstream_port_const or
bt_connection_borrow_upstream_port_const to return NULL/None, as
connections always have both ports set.

In field.py, the call in _create_field_from_ptr after
bt_field_borrow_class_const can be removed, as it's impossible for this
function to legitimately return NULL/None.

The call in _VariantField.selected_option is unnecessary, as it is after
a pre-condition check that the variant's selected option is non-NULL.
If we want to make Python bindings user-friendly (avoid hitting
pre-condition checks), we check before the call if the variant has a
currently selection option, and raise ValueError otherwise.  However,
there does not seem to be an API for that at the moment.

Change-Id: I585e57030d58256cbf51161e75394503c281addf
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1743
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>