Kube-proxy's iptables mode used to care whether utiliptables's
EnsureRule was able to use "iptables -C" or if it had to implement it
hackily using "iptables-save". But that became irrelevant when
kube-proxy was reimplemented using "iptables-restore", and no one ever
noticed. So remove that check.
ipvs `getProxyMode` test fails on mac as `utilipvs.GetRequiredIPVSMods`
try to reach `/proc/sys/kernel/osrelease` to find version of the running
linux kernel. Linux kernel version is used to determine the list of required
kernel modules for ipvs.
Logic to determine kernel version is moved to GetKernelVersion
method in LinuxKernelHandler which implements ipvs.KernelHandler.
Mock KernelHandler is used in the test cases.
Read and parse file is converted to go function instead of execing cut.
These are based on recommendation from
[staticcheck](http://staticcheck.io/).
- Removes dead type/function along with the import that the function
introduced.
- Removes unused struct fields.
- Removes select nested in a tight for loop, the select does not have a
default, so it will be blocking.
