remove HAVE_DUP #1072

Vim defines this for Windows, so there's no Neovim-supported system for
which this would not be defined.

src/nvim/fileio.c

@@ -1755,13 +1755,11 @@ failed:
 #endif
   xfree(buffer);
 
-#ifdef HAVE_DUP
   if (read_stdin) {
     /* Use stderr for stdin, makes shell commands work. */
     close(0);
     ignored = dup(2);
   }
-#endif
 
   if (tmpname != NULL) {
     os_remove((char *)tmpname);  // delete converted file

src/nvim/os/os_defs.h

@@ -103,9 +103,6 @@
 # include <strings.h>
 #endif
 
-// For dup(3).
-#define HAVE_DUP
-
 /// Function to convert -errno error to char * error description
 ///
 /// -errno errors are returned by a number of os functions.