tests: net: Remove CONFIG_NET_HEADERS_ALWAYS_CONTIGUOUS assignment
tests: net: Remove CONFIG_NET_HEADERS_ALWAYS_CONTIGUOUS assignment

This symbol is not user-configurable (has no prompt), so assignments in
prj.conf have no effect on it.

(It has no defaults either, and isn't selected/implied, so nothing in
vanilla Zephyr will ever enable it.)

Signed-off-by: Ulf Magnusson <01743f1c1deb1fe75271d62edb65757d98ac0bcb@nordicsemi.no>

nashif/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,nashif/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr