Removed unneeded step in installation.md

In the "Clearing the defaults" part of the guide shown below, there is no need to remove the `www/css/app.css` file.
Since the project created by `$ ionic start hello` does not create it anymore.

![screen shot 2014-04-24 at 22 46 01](https://cloud.githubusercontent.com/assets/520376/2797794/f43a6f0a-cc3c-11e3-95d3-d38507bf4234.png)


The current folder structure is in the screenshot below; it shows that the css folder now just contains an empty "style.css" file after creating the project.
![screen shot 2014-04-24 at 22 39 52](https://cloud.githubusercontent.com/assets/520376/2797775/153b335c-cc3c-11e3-8746-c2fb9d8e5761.png)