um: Remove unused PGD_BOUND macro

It's no longer used since commit 11100b1 ("uml: delete
unused code").

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Link: https://patch.msgid.link/20241128083137.2219830-8-tiwei.btw@antgroup.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

Author: btw616

arch/um/os-Linux/main.c

@@ -19,7 +19,6 @@
 #include <um_malloc.h>
 #include "internal.h"
 
-#define PGD_BOUND (4 * 1024 * 1024)
 #define STACKSIZE (8 * 1024 * 1024)
 #define THREAD_NAME_LEN (256)