This was implemented partly in server.go and partly in
server_others.go even though even the parts in server.go were totally
linux-specific. Simplify things by putting it all in server_others.go
and get rid of some unnecessary abstraction.

Back when iptables was first made the default, there were
theoretically some users who wouldn't have been able to support it due
to having an old /sbin/iptables. But kube-proxy no longer does the
things that didn't work with old iptables, and we removed that check a
long time ago. There is also a check for a new-enough kernel version,
but it's checking for a feature which was added in kernel 3.6, and no
one could possibly be running Kubernetes with a kernel that old. So
the fallback code now never actually falls back, so it should just be
removed.

If the user passes "--proxy-mode ipvs", and it is not possible to use
IPVS, then error out rather than falling back to iptables.

There was never any good reason to be doing fallback; this was
presumably erroneously added to parallel the iptables-to-userspace
fallback (which only existed because we had wanted iptables to be the
default but not all systems could support it).

In particular, if the user passed configuration options for ipvs, then
they presumably *didn't* pass configuration options for iptables, and
so even if the iptables proxy is able to run, it is likely to be
misconfigured.

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