Xcode project: remove Wesnoth_Prefix.pch from 'Prefix Header' setting

Apparently it is not needed any more and this removes the last compiler
warning.
 Xcode project: don't show warnings for third-party code (release build)

This was already done for the debug build in commit
8567a0d616d323cc74c61fd5d8ef83e025f22c42, added for the release build
as well here.