This removes the custom `help` implementation from `DevCommand` in favor
of Thor's `help` implementation.

__Before__

  ```console
  $ bin/rails dev --help
  bin/rails dev:cache # Toggle development mode caching on/off.

  $ bin/rails dev:cache --help
  bin/rails dev:cache # Toggle development mode caching on/off.
  ```

__After__

  ```console
  $ bin/rails dev --help
  Commands:
    bin/rails dev:cache           # Toggle development mode caching on/off
    bin/rails dev:help [COMMAND]  # Describe available commands or one specific command

  $ bin/rails dev:cache --help
  Usage:
    bin/rails dev:cache

  Toggle development mode caching on/off
  ```