Xcode: Don't set TARGET_BUILD_DIR; that's handled by a copy phase

This reverts an earlier change that tried to fix the relationship between
Qt's concept of output directories with what Xcode expects, but it broke
DESTDIR. The relationship between Qt and Xcode is still a mess, but at
least DESTDIR now works.

Change-Id: I44f056d48c87359a609e0337da266120ba4eb155
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Andy Shaw <andy.shaw@digia.com>