configfs: remove mentions of committable items

A proposition of implementation of committable items has been rejected
due to the gpio-sim module being the only user and configfs not getting
much development in general. In that case, let's remove the notion
of committable items from docs and headers.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>