ARC: nsim_{700|700be|hs38be}_defconfigs: Disable networking

We don't have yet any brc700 or big-enadian platforms with networking
support to run this particular configuration.

Whenever QEMU for ARC supports arc700 or big-endian targets we may revisit
this one.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>