devicetree: remove deprecated DT_CHOSEN_*_LABEL macros

DT_CHOSEN_ZEPHYR_FLASH_CONTROLLER_LABEL and
DT_CHOSEN_ZEPHYR_ENTROPY_LABEL were deprecated a couple of releases ago,
so it is time to remove them. Note that the zephyr/devicetree/zephyr.h
is also removed, since both macros were its last content.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>