samples/shields/npm6001_ek: Remove unnecessary header

To use the shell one does not need anymore to pull unistd.h,
if one uses sys_getopt, we need to include sys/sys_getopt.h

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>

samples/shields/npm6001_ek/src/main.c

@@ -11,10 +11,8 @@
 #include <zephyr/drivers/watchdog.h>
 #include <zephyr/dt-bindings/gpio/nordic-npm6001-gpio.h>
 #include <zephyr/dt-bindings/regulator/npm6001.h>
-#include <zephyr/posix/unistd.h>
 #include <zephyr/shell/shell.h>
 #include <zephyr/sys/printk.h>
-
 #include <zephyr/sys/sys_getopt.h>
 
 struct regulators_map {