remove deprecated etcd backend

This allows us to remove the manual replace directives
github.com/dgrijalva/jwt-go and google.golang.org/grpc, so that we can
remove the CVE warnings and update the grpc packages.

While the etcdv3 backend is also marked as deprecated, the changes here
are done in a manner to keep that backend working for the time being.

tools/tools.go

@@ -5,7 +5,6 @@ package tools
 
 import (
 	_ "github.com/golang/mock/mockgen"
-	_ "github.com/golang/protobuf/protoc-gen-go"
 	_ "github.com/mitchellh/gox"
 	_ "github.com/nishanths/exhaustive"
 	_ "golang.org/x/tools/cmd/cover"