tests: net: all: kconfig: Do not assign promptless NET_INIT_PRIO

Assignments have no effect on promptless symbols. Flagged by
https://github.com/zephyrproject-rtos/zephyr/pull/20742

.

NET_INIT_PRIO (defined in subsys/net/ip/Kconfig) is currently hardcoded
to 90 (it's not set via any Kconfig.defconfig files either). Maybe it
could be given a prompt or be removed, but just remove the assignment
for now.

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