samples: sensor: remove fxos8700-hid

This sample uses the FXOS8700 compatible accelerometer to emulate
relative mouse movement. While this is an interesting example, it does
not provide anything unique in terms of sensor drivers or USB device
support. The example has no documentation and unsafe report buffer
handling. The example would need to be completely rewritten to make it
generic and use any accelerometer, e.g. with an accel0 alias. From a USB
perspective, there is no interest in maintaining this sample or porting
it to the new device support and new HID device API.

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