Replace deprecated linter (#2176)

Upstream is replacing `golint` with `revive`.
* Cleanup unused mixin go files.

Signed-off-by: Ben Kochie <superq@gmail.com>

.golangci.yml

@@ -1,6 +1,6 @@
 linters:
   enable:
-    - golint
+    - revive
   disable:
     # Disable soon to deprecated[1] linters that lead to false
     # positives when build tags disable certain files[2]

docs/node-mixin/go.mod

@@ -1,9 +0,0 @@
-module github.com/prometheus/node_exporter/docs/node-mixin
-
-go 1.15
-
-require (
-	github.com/google/go-jsonnet v0.16.0
-	github.com/jsonnet-bundler/jsonnet-bundler v0.4.0
-	github.com/prometheus/prometheus v1.8.2-0.20200805170718-983ebb4a5133
-)

docs/node-mixin/tools.go

@@ -1,27 +0,0 @@
-// Copyright 2020 The prometheus Authors
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//     http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-//go:build tools
-// +build tools
-
-// Package tools tracks dependencies for tools that used in the build process.
-// See https://github.com/golang/go/issues/25922
-package tools
-
-import (
-	_ "github.com/google/go-jsonnet/cmd/jsonnet"
-	_ "github.com/google/go-jsonnet/cmd/jsonnetfmt"
-	_ "github.com/jsonnet-bundler/jsonnet-bundler/cmd/jb"
-	_ "github.com/prometheus/prometheus/cmd/promtool"
-)