EXT-1651 Fix "You cannot close the Buy Land window" error message that...

EXT-1651 Fix "You cannot close the Buy Land window" error message that prevents buying land more than once per login session.
Made LLFloaterBuyLandUI destroy itself on close, which cleans up pending currency transactions.
Changed LLViewerParcelMgr::deleteBuyInfo() signature because I found it confusing.
Reviewed with Richard.