drivers: sensor: Remove unused function

Building with clang warns:

drivers/sensor/st/lsm6dso16is/lsm6dso16is.c:83:19: error: unused
function 'lsm6dso16is_reboot' [-Werror,-Wunused-function]
static inline int lsm6dso16is_reboot(const struct device *dev)
                  ^

Signed-off-by: Tom Hughes <tomhughes@chromium.org>