Don't overwrite bootstrap error, success and information alert colors

http://getbootstrap.com/2.3.2/components.html#alerts

git blame doesn't reveal any useful information as to why this was added in
the first place.

(Edit: added screenshots)

Before:
![before](https://f.cloud.github.com/assets/133575/2399029/3c827500-a9fb-11e3-9029-0665d16f9249.png)
After:
![after](https://f.cloud.github.com/assets/133575/2399031/40fd885e-a9fb-11e3-828d-b5001dc5bd7e.png)