This is no longer used anywhere.

Fixes #5588.

Create a new, seperate RuboCop file that uses our RSpec rule
configuration if a specified tap has a `spec` directory.

Also, while we're here, fix up the various rules so that we don't have
any outdated TODOs, disable some rules for good and fix `brew style`
for Homebrew/bundle.

This provides a more consistent version for `rubocop` than relying on
`Homebrew.install_gem_setup_path!` (and we really want `brew style` to
provide consistent output).

If you are using en external rubocop binary you will encounter the
following error.

.rubocop_todo.yml: RSpec/FilePath has the wrong namespace - should be Rails

By moving the option to the config file an external rubocop would be
treated equally to  brew style  without having to supply additional
command line options.

Use 124 max line length everywhere. Also, reduce tap max line length to
189 as Homebrew/homebrew-core has that as a maximum now. In future
Homebrew/homebrew-core will also be reduced to 124 maximum line length.