Remove now-unnecessary build.go

Caddy can be built, even with plugins, without modifying the source
code and without special build scripts, thanks to Go modules. See
the README or wiki.

caddy/build.go

-// Copyright 2015 Light Code Labs, LLC
-//
-// 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.
-
-// +build dev
-
-// build.go automates proper versioning of caddy binaries.
-// Use it like:   go run build.go
-// You can customize the build with the -goos, -goarch, and
-// -goarm CLI options:   go run build.go -goos=windows
-//
-// To get proper version information, this program must be
-// run from the directory of this file, and the source code
-// must be a working git repository, since it needs to know
-// if the source is in a clean state.
-//
-// This program is NOT required to build Caddy from source
-// since it is go-gettable. (You can run plain `go build`
-// in this directory to get a binary.) However, issues filed
-// without version information will likely be closed.
-package main
-
-import (
-	"flag"
-	"fmt"
-	"log"
-	"os"
-	"os/exec"
-	"path/filepath"
-
-	"github.com/caddyserver/builds"
-)
-
-var goos, goarch, goarm string
-var race bool
-
-func init() {
-	flag.StringVar(&goos, "goos", "", "GOOS for which to build")
-	flag.StringVar(&goarch, "goarch", "", "GOARCH for which to build")
-	flag.StringVar(&goarm, "goarm", "", "GOARM for which to build")
-	flag.BoolVar(&race, "race", false, "Enable race detector")
-}
-
-func main() {
-	flag.Parse()
-
-	gopath := os.Getenv("GOPATH")
-
-	pwd, err := os.Getwd()
-	if err != nil {
-		log.Fatal(err)
-	}
-
-	ldflags, err := builds.MakeLdFlags(filepath.Join(pwd, ".."))
-	if err != nil {
-		log.Fatal(err)
-	}
-
-	args := []string{"build", "-ldflags", ldflags}
-	args = append(args, "-asmflags", fmt.Sprintf("-trimpath=%s", gopath))
-	args = append(args, "-gcflags", fmt.Sprintf("-trimpath=%s", gopath))
-	if race {
-		args = append(args, "-race")
-	}
-	cmd := exec.Command("go", args...)
-	cmd.Stderr = os.Stderr
-	cmd.Stdout = os.Stdout
-	cmd.Env = os.Environ()
-	for _, env := range []string{
-		"CGO_ENABLED=0",
-		"GOOS=" + goos,
-		"GOARCH=" + goarch,
-		"GOARM=" + goarm,
-	} {
-		if race && env == "CGO_ENABLED=0" {
-			continue
-		}
-		cmd.Env = append(cmd.Env, env)
-	}
-
-	err = cmd.Run()
-	if err != nil {
-		log.Fatal(err)
-	}
-}