Remove unused user_key parameter from a few functions.

Every caller was passing HKEY_CURRENT_USER.
TBR=rogerta
Review URL: https://codereview.appspot.com/5789043

git-svn-id: http://rlz.googlecode.com/svn/trunk@71 10bc0f33-e4bf-9a86-80cf-af638054f0c4