um: Remove unused THREAD_NAME_LEN macro

It's no longer used since commit 42fda66 ("uml: throw out
CONFIG_MODE_TT").

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Link: https://patch.msgid.link/20241128083137.2219830-9-tiwei.btw@antgroup.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

Author: btw616

arch/um/os-Linux/main.c

@@ -20,7 +20,6 @@
 #include "internal.h"
 
 #define STACKSIZE (8 * 1024 * 1024)
-#define THREAD_NAME_LEN (256)
 
 long elf_aux_hwcap;