Prevent confidential data requests from appearing

 in the users activity stats - Redmine #1478
this is wrong, we don't need to select all fields, we actually don't need to select any fields, only COUNT() is what we need.
But this isn't performance issue, ->fetchField returns reply.id of first reply made by this user instead of number of comments.
e.g. /users/amandasmith posted only 2 comments but comments counter shows 2831