include: zephyr: remove __ZEPHYR__

CMake always injects __ZEPHYR__ to any code being built on Zephyr (e.g.
a module, an application, etc.). Requiring an include that defines
__ZEPHYR__ to determine if I'm on Zephyr is problematic as well: it
requires to include a Zephyr header (which will exist if building for...
Zephyr!).

Note that this change leaves <zephyr/zephyr.h> in a questionable
position: why shouldn't I just include <zephyr/kernel.h>?

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