patch 7.4.1399

Problem:    The MS-DOS code does not build.
Solution:   Remove the old MS-DOS code.

src/window.c

@@ -6438,7 +6438,7 @@ vim_FullName(
 	/* something failed; use the file name (truncate when too long) */
 	vim_strncpy(buf, fname, len - 1);
     }
-#if defined(MACOS_CLASSIC) || defined(MSDOS) || defined(MSWIN)
+#if defined(MACOS_CLASSIC) || defined(MSWIN)
     slash_adjust(buf);
 #endif
     return retval;