treewide: remove helpers for opening chips by name & number

Helper wrappers around gpiod_chip_open() shouldn't really be part of
the core, low-level library. They assume that devtmpfs is mounted at
/dev and that GPIO chips follow the gpiochipX naming convention.

This changeset removes all variants of gpiod_chip_open() other than
the one taking the path as argument. We're doing this treewide so
C++ and Python bindings lose the 'how' argument that currently allows
to change the way the chips are opened by the chip's constructors or
the open() method in C++.

The gpio-tools programs still support opening chips by number, name
and path but they implement this functionality locally rather than
using the library code.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>