enable full linter across the codebase (#9620)

enable linter using golangci-lint across
codebase to run a bunch of linters together,
we shall enable new linters as we fix more
things the codebase.

This PR fixes the first stage of this
cleanup.

cmd/config/etcd/dns/types.go

@@ -23,7 +23,6 @@ import (
 
 const (
 	defaultTTL            = 30
-	defaultPrefixPath     = "/skydns"
 	defaultContextTimeout = 5 * time.Minute
 )