Remove (Open)Watcom support.

This compiler is not being developed since several years and almost certainly
can't be used to build the current wxWidgets sources anyhow, so remove all
support for it, including a lot of extremely ugly workarounds for its bugs
with template functions.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76546 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/file.cpp

@@ -56,9 +56,7 @@
         #include "wx/msw/wrapwin.h"
     #endif
 #elif defined(__DOS__)
-    #if defined(__WATCOMC__)
-       #include <io.h>
-    #elif defined(__DJGPP__)
+    #if defined(__DJGPP__)
        #include <io.h>
        #include <unistd.h>
        #include <stdio.h>