* Mediators work to remove al Qaeda presence
* Tribesmen said to lay down arms after amnesty offer
* U.S. urging all sides to find negotiated way out
By Suadad al-Salhy
BAGHDAD, Jan 10 (Reuters) - The prospect of an imminent Iraqi army assault on Falluja receded on Friday as negotiators tried to work out a deal under which al Qaeda militants who seized the city 10 days ago would ...