cli: drop invalid validation for default-routes in nmcli

Currently, default-routes cannot be added like regular static-routes
as ipv4.routes setting.
Instead, one has to configure "ipv4.gateway" and "ipv4.never-default".
That of course should be fixed, for example to configure a default-route
in different routing tables.

As it is, both nmcli's parse function and libnm's
NMSettingIPConfig:verify() functions reject default-routes.

But nmcli goes way beyond that, it also rejects all networks with
"0.0.0.0"/"::" even if their prefix length is not zero. Such routes are
not default-routes, and nmcli has no business rejecting them. The
correct way for checking for a default-route is to check the prefix-length
for zero.

Drop the wrong validation in nmcli.

Note, it may still not be the best idea to add catch-all routes like
"0.0.0.0/1" and "128.0.0.0/1". It just defeats what counts as a default-route.
NM has other means (like configuring the route-metric) to handle routing
in face of multiple interfaces. But sure, whatever works for you.
7 jobs for th/nmcli-validate-default-route
Status Job ID Name Coverage
  Test
canceled #100374
allowed to fail
checkpatch

canceled #100375
f28_build_autotools

canceled #100376
f28_build_meson

canceled #100377
f28_rpm_autotools_dbg

canceled #100378
f28_rpm_meson_dbg

canceled #100379
f_latest_build_autotools

canceled #100380
allowed to fail
f_rawhide_build_autotools