Stop using private browsing

This allows the cookie keywords to work with the default Firefox profile.
Now testing that app sees cookies set by Selenium and Selenium sees cookies set by app (both failed before the profile update in this PR). Remove server sending cookies that is no longer needed since the previous revision of the suite.

A lot of the effects of private browsing happen anyways because Selenium creates a temporary profile at session start.
Private browsing was enabled in a commit to stop Firefox from opening two browser windows, and this change does not cause a regression on my system. I wonder if @yanne remembers why this specific setting was made (many preferences changed in the commit).
[Commit](https://code.google.com/p/robotframework-seleniumlibrary/source/detail?r=58107cafeb5453540b471a95d2b3f65b03afbaed&path=/src/SeleniumLibrary/firefoxprofile/prefs.js) that enabled private browsing and [related issue](https://code.google.com/p/robotframework-seleniumlibrary/issues/detail?id=171)

Fixes #177