samples: hid-mouse: rework sample to use input subsys

Overall, this makes the sample code much simpler and smaller.
Whit the changes introduced in commit commit 57e0da4d
("boards: add zephyr,code properties to the various gpio-keys nodes")'
this example will work out of the box with any board that has at least
zephyr,code = <INPUT_KEY_0>; in its devicetree.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>