Migrate to go1.12 to simplify our cmd/http package (#7302)

Simplify the cmd/http package overall by removing
custom plain text v/s tls connection detection, by
migrating to go1.12 and choose minimum version
to be go1.12

Also remove all the vendored deps, since they
are not useful anymore.

main.go

@@ -1,3 +1,5 @@
+// +build go1.12
+
 /*
  * Minio Cloud Storage, (C) 2016, 2017, 2018 Minio, Inc.
  *
@@ -23,49 +25,14 @@
 package main // import "github.com/minio/minio"
 
 import (
-	"fmt"
 	"os"
-	"runtime"
 
-	version "github.com/hashicorp/go-version"
-	"github.com/minio/mc/pkg/console"
 	minio "github.com/minio/minio/cmd"
 
 	// Import gateway
 	_ "github.com/minio/minio/cmd/gateway"
 )
 
-const (
-	// Minio requires at least Go v1.9.4
-	minGoVersion        = "1.9.4"
-	goVersionConstraint = ">= " + minGoVersion
-)
-
-// Check if this binary is compiled with at least minimum Go version.
-func checkGoVersion(goVersionStr string) error {
-	constraint, err := version.NewConstraint(goVersionConstraint)
-	if err != nil {
-		return fmt.Errorf("'%s': %s", goVersionConstraint, err)
-	}
-
-	goVersion, err := version.NewVersion(goVersionStr)
-	if err != nil {
-		return err
-	}
-
-	if !constraint.Check(goVersion) {
-		return fmt.Errorf("Minio is not compiled by go %s. Minimum required version is %s, go %s release is known to have security issues. Please recompile accordingly", goVersionConstraint, minGoVersion, runtime.Version()[2:])
-	}
-
-	return nil
-}
-
 func main() {
-	// When `go get` is used minimum Go version check is not triggered but it would have compiled it successfully.
-	// However such binary will fail at runtime, hence we also check Go version at runtime.
-	if err := checkGoVersion(runtime.Version()[2:]); err != nil {
-		console.Errorln(err)
-	}
-
 	minio.Main(os.Args)
 }