caddytls: Change clustering to be a plugin to the caddytls package (#2459)

* caddytls: Change clustering to be a plugin to the caddytls package

Should resolve the failure in
https://github.com/coredns/coredns/pull/2541.

This change is breaking to clustering plugin developers (not Caddy
users), but logical, since only the caddytls package uses CertMagic
directly (the httpserver package also uses it, but only because it also
uses the caddytls plugin); and it is early enough that no clustering
plugins really exist yet.

This will also require a change of devportal
so that it looks for a different registration function, which has moved
to the caddytls package.

* Remove unused variable

* caddyhttp: Fix test (adjust plugin counting)

* ummmm, remove extra line break

somehow VS Code didn't fmt on save... weird.

plugins.go

@@ -22,7 +22,6 @@ import (
 	"sync"
 
 	"github.com/mholt/caddy/caddyfile"
-	"github.com/mholt/certmagic"
 )
 
 // These are all the registered plugins.
@@ -107,11 +106,6 @@ func ListPlugins() map[string][]string {
 		p["caddyfile_loaders"] = append(p["caddyfile_loaders"], defaultCaddyfileLoader.name)
 	}
 
-	// cluster plugins in registration order
-	for name := range clusterProviders {
-		p["clustering"] = append(p["clustering"], name)
-	}
-
 	// List the event hook plugins
 	eventHooks.Range(func(k, _ interface{}) bool {
 		p["event_hooks"] = append(p["event_hooks"], k.(string))
@@ -456,21 +450,6 @@ func loadCaddyfileInput(serverType string) (Input, error) {
 	return caddyfileToUse, nil
 }
 
-// ClusterPluginConstructor is a function type that is used to
-// instantiate a new implementation of both certmagic.Storage
-// and certmagic.Locker, which are required for successful
-// use in cluster environments.
-type ClusterPluginConstructor func() (certmagic.Storage, error)
-
-// clusterProviders is the list of storage providers
-var clusterProviders = make(map[string]ClusterPluginConstructor)
-
-// RegisterClusterPlugin registers provider by name for facilitating
-// cluster-wide operations like storage and synchronization.
-func RegisterClusterPlugin(name string, provider ClusterPluginConstructor) {
-	clusterProviders[name] = provider
-}
-
 // OnProcessExit is a list of functions to run when the process
 // exits -- they are ONLY for cleanup and should not block,
 // return errors, or do anything fancy. They will be run with