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

src/common/textbuf.cpp

@@ -36,7 +36,7 @@
 // default type is the native one
 
 const wxTextFileType wxTextBuffer::typeDefault =
-#if defined(__WINDOWS__) || defined(__DOS__)
+#if defined(__WINDOWS__)
   wxTextFileType_Dos;
 #elif defined(__UNIX__)
   wxTextFileType_Unix;