Revert "Fall back to network boot as the last possible boot option"

This reverts commit 94ca5a985919cca312c505bbb0c824d175cc6bb8.
diff --git a/vl.c b/vl.c
index 3057f9d..e57f58f 100644
--- a/vl.c
+++ b/vl.c
@@ -4572,7 +4572,7 @@
     int snapshot, linux_boot, net_boot;
     const char *initrd_filename;
     const char *kernel_filename, *kernel_cmdline;
-    char boot_devices[33] = "cadn"; /* default to HD->floppy->CD-ROM->Network */
+    char boot_devices[33] = "cad"; /* default to HD->floppy->CD-ROM */
     DisplayState *ds;
     DisplayChangeListener *dcl;
     int cyls, heads, secs, translation;