patch 9.0.0287: Irix systems no longer exist

Problem:    Irix systems no longer exist.
Solution:   Remove references to Irix. (Yegappan Lakshmanan, closes #10994)

src/filepath.c

@@ -4071,10 +4071,8 @@ addfile(
     /*
      * Append a slash or backslash after directory names if none is present.
      */
-#ifndef DONT_ADD_PATHSEP_TO_DIR
     if (isdir && (flags & EW_ADDSLASH))
 	add_pathsep(p);
-#endif
     ((char_u **)gap->ga_data)[gap->ga_len++] = p;
 }