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.