tests: drop the .dispose() method from gpiosim-glib's chip class

tests: drop the .dispose() method from gpiosim-glib's chip class

The .dispose() method should be used to drop references to any other
GObjects while .finalize() should free any remaining memory. While we are
dropping references in .dispose(), these are references to the libgpiosim
objects that are also guaranteed to always be at most equal 1. Move the
reference drops to .finalize() and remove .dispose() entirely.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>