Remove remaining MSDOS support code

The remaining DOS source files where already removed in
26a0a24f38. This removes the remaining __DOS__
and __DJGPP__ (the DOS gcc) code paths.

Closes https://github.com/wxWidgets/wxWidgets/pull/100

include/wx/filename.h

@@ -28,7 +28,7 @@ class WXDLLIMPEXP_FWD_BASE wxFFile;
 
 // this symbol is defined for the platforms where file systems use volumes in
 // paths
-#if defined(__WINDOWS__) || defined(__DOS__)
+#if defined(__WINDOWS__)
     #define wxHAS_FILESYSTEM_VOLUMES
 #endif