Do not pass raw user data to unserialize

Filters were moved to TOKEN api, so the code in current_user_api to handle
?filter= on URL query strings is a left over from this move and is no
longer necessary.

This issue was reported by Matthias Karlsson (http://mathiaskarlsson.me)
as part of Offensive Security's bug bounty program [1].

Fixes #17875

[1] http://www.offensive-security.com/bug-bounty-program/

Signed-off-by: Damien Regad <dregad@mantisbt.org>