Core/Trade: Correction of distance checking

The check in Player::UpdatePosition is wrong, as it halves the available distance set by the client. Other bug is when trade is cancelled due to moving out of range the "Trade cancelled." message appears twice. Client checks the distance by itself (and closes the trade window properly w/o additional server-side actions) so I think it's enough to check the distance when trade is initiated / accepted.

How to reproduce is easy, begin trade at maximum range allowed by client and move closer to your target. It gets cancelled. 

*10.0f -> 11.1f*
http://www.wowwiki.com/API_CheckInteractDistance