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 <Ulf.Magnusson@nordicsemi.no>