Remove plugin registration generation step

This change removes the generation of command/plugin.go so that plugin
registration can be managed manually. As we begin to break out plugins
we will need to begin modifying this file, possibly with stubs to allow
for the removal of plugins without breaking Packer's backwards
  compatibility with 1.7.0. After 1.8.0 is released we should be able to
  revet these changes so that we can continue to generate the plugin
  file for those plugins core to Packer.

command/plugin.go

@@ -1,7 +1,3 @@
-//
-// This file is automatically generated by scripts/generate-plugins.go -- Do not edit!
-//
-
 package command
 
 import (

main.go

@@ -1,6 +1,5 @@
 // This is the main package for the `packer` application.
 
-//go:generate go run ./scripts/generate-plugins.go
 package main
 
 import (