Helm cleanup

The code of prelude-helm module, with ugly formatted code and many typo and grammar mistakes, is really a mess. I know helm is a gigantic package, but it does not require that much code to configure. Many options are not necessary and most of the keys are subjected to personal taste. I think most of the code in the helm module is too personal to make it a part of prelude, we should leave most of the customisable options to the users, and have a bare minimal working configuration like this one:

https://gist.github.com/toctan/ac523ef379f14f5486e2 

In this pull request, I removed all the options which are already the default and thus are not necessary and fixed some typos.
Helm already bind these functions and keys:
```el
(define-key map (kbd "C-z") 'helm-execute-persistent-action)
(define-key map (kbd "C-i") 'helm-select-action)
```

@tuhdo Why do you swap these two original helm bindings? For me open a menu to select action with <kbd>tab<kbd> key makes sense but <kbd>C-z</kbd> does not.