Fixed wxFileDialog assertion failure under wxOSX Cocoa.

Using wxFD_FILE_MUST_EXIST in combination with an extra panel in wxFileDialog wasn't working properly. Simply remove the unused flag under OS X. Caused problems with Cocoa only (not Carbon), and at least under 10.6 and 10.8.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74971 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775