Rely on Thor for encrypted command help

[jonathanhefner]

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

Before

$ bin/rails encrypted --help
Usage:
  bin/rails encrypted

=== Storing Encrypted Files in Source Control

The Rails `encrypted` commands provide access to encrypted files or configurations.
...

$ bin/rails encrypted:edit --help
Usage:
  bin/rails encrypted

=== Storing Encrypted Files in Source Control

The Rails `encrypted` commands provide access to encrypted files or configurations.
...

After

$ bin/rails encrypted --help
Commands:
  bin/rails encrypted:edit            # Open the decrypted file in `$EDITOR` for editing
  bin/rails encrypted:help [COMMAND]  # Describe available commands or one specific command
  bin/rails encrypted:show            # Show the decrypted contents of the file

Options:
  -k, [--key=KEY]  # The Rails.root relative path to the encryption key
                   # Default: config/master.key

=== Storing Encrypted Files in Source Control

The Rails `encrypted` commands provide access to encrypted files or configurations.
...

$ bin/rails encrypted:edit --help # OR bin/rails encrypted:help edit
Usage:
  bin/rails encrypted:edit

Options:
  -k, [--key=KEY]  # The Rails.root relative path to the encryption key
                   # Default: config/master.key

Open the decrypted file in `$EDITOR` for editing