Remove old thread_local.h

This was to work around a clang 9.0.0 bug.  We are on clang 13.  Make
things simpler by removing it.

Change-Id: I4bad16270795bb82b8c498146a4bf7388565a1c7
Signed-off-by: Austin Schuh <austin.linux@gmail.com>
diff --git a/aos/libc/aos_strsignal.cc b/aos/libc/aos_strsignal.cc
index b46e46e..12838b4 100644
--- a/aos/libc/aos_strsignal.cc
+++ b/aos/libc/aos_strsignal.cc
@@ -4,10 +4,8 @@
 
 #include "glog/logging.h"
 
-#include "aos/thread_local.h"
-
 const char *aos_strsignal(int signal) {
-  AOS_THREAD_LOCAL char buffer[512];
+  thread_local char buffer[512];
 
   if (signal >= SIGRTMIN && signal <= SIGRTMAX) {
     CHECK_GT(snprintf(buffer, sizeof(buffer), "Real-time signal %d",