Use the same wxDEBUG_LEVEL settings for Xcode projects as elsewhere.

Use the same debug flag default settings in the build/osx Xcode projects
as are used by all other makefiles and projects - that is,
wxDEBUG_LEVEL=1.  This is done automatically by the headers, no need to
define any extra symbols when building.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73773 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775