Drop --start-maximized option

This option was a workaround for Windows-specific issue
https://code.google.com/p/chromium/issues/detail?id=151836

It does not seem needed anymore and it creates odd experience when launching tests.

I am not using Windows as main development platform but I have tested with a minimal karma setup on Windows 7 with latest Chrome stable version (34.0.1847.131). I could not trigger that issue after having dropped the option: Chrome is normally launched (visible and not maximized).

Perhaps to be confirmed on other (Windows) systems before merging this PR.