Remove unfinished command infrastructure.

If we're gonna do this right, it will look mighty different from this anyway.
(Looking at you, Rails 5.1).

It isn't being used in any code as of now, so yanking is the best option.

railties/lib/rails/command.rb

-require 'rails/commands/commands_tasks'
-
-module Rails
-  class Command #:nodoc:
-    attr_reader :argv
-
-    def initialize(argv = [])
-      @argv = argv
-
-      @option_parser = build_option_parser
-      @options = {}
-    end
-
-    def self.run(task_name, argv)
-      command_name = command_name_for(task_name)
-
-      if command = command_for(command_name)
-        command.new(argv).run(command_name)
-      else
-        Rails::CommandsTasks.new(argv).run_command!(task_name)
-      end
-    end
-
-    def run(command_name)
-      parse_options_for(command_name)
-      @option_parser.parse! @argv
-
-      public_send(command_name)
-    end
-
-    def self.options_for(command_name, &options_to_parse)
-      @@command_options[command_name] = options_to_parse
-    end
-
-    def self.set_banner(command_name, banner)
-      options_for(command_name) { |opts, _| opts.banner = banner }
-    end
-
-    private
-      @@commands = []
-      @@command_options = {}
-
-      def parse_options_for(command_name)
-        @@command_options.fetch(command_name, proc {}).call(@option_parser, @options)
-      end
-
-      def build_option_parser
-        OptionParser.new do |opts|
-          opts.on('-h', '--help', 'Show this help.') do
-            puts opts
-            exit
-          end
-        end
-      end
-
-      def self.inherited(command)
-        @@commands << command
-      end
-
-      def self.command_name_for(task_name)
-        task_name.gsub(':', '_').to_sym
-      end
-
-      def self.command_for(command_name)
-        @@commands.find do |command|
-          command.public_instance_methods.include?(command_name)
-        end
-      end
-  end
-end

railties/lib/rails/commands.rb

@@ -13,6 +13,6 @@
 command = ARGV.shift
 command = aliases[command] || command
 
-require 'rails/command'
+require 'rails/commands/commands_tasks'
 
-Rails::Command.run(command, ARGV)
+Rails::CommandsTasks.new(ARGV).run_command!(command)