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/file.cpp

@@ -53,14 +53,6 @@
     #ifdef __GNUWIN32__
         #include "wx/msw/wrapwin.h"
     #endif
-#elif defined(__DOS__)
-    #if defined(__DJGPP__)
-       #include <io.h>
-       #include <unistd.h>
-       #include <stdio.h>
-    #else
-        #error  "Please specify the header with file functions declarations."
-    #endif
 #elif (defined(__WXSTUBS__))
     // Have to ifdef this for different environments
     #include <io.h>
@@ -483,7 +475,7 @@ bool wxFile::Eof() const
 
     wxFileOffset iRc;
 
-#if defined(__DOS__) || defined(__UNIX__) || defined(__GNUWIN32__)
+#if defined(__UNIX__) || defined(__GNUWIN32__)
     // @@ this doesn't work, of course, on unseekable file descriptors
     wxFileOffset ofsCur = Tell(),
     ofsMax = Length();