drivers: sensor: Remove unused function

Building with clang warns:

drivers/sensor/st/ism330dhcx/ism330dhcx.c:107:19: error: unused function
'ism330dhcx_reboot' [-Werror,-Wunused-function]
static inline int ism330dhcx_reboot(const struct device *dev)

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