clean up go.mod, get rid of client-go v1.5.2

[xrstf]

client-go 1.5.2 has been retracted and having it here as a dependency forces not just Multus, but everyone who imports Multus to do the same replace dance with tons of Kube dependencies.

From what I can see, there is simply no need to replace tons of dependencies. So I cleaned it up and go mod tidy and go build are happy.

[dougbtv]

Thanks for the heads up and contribution on this one!

[s1061123]

LGTM.

[coveralls]

coverage: 56.454% (-7.4%) from 63.857%
when pulling 808185b on xrstf:master
into e1a0d2a on k8snetworkplumbingwg:master.