Revert "Cause Fatal error when invalid pthread_id is detected."

Some code like in https://buganizer.corp.google.com/u/0/issues/19942911 need to change first.

This reverts commit 03324780aae9ff28c8acf52debf0ea39120e5ab8.

Change-Id: I13ff1e5b3d0672bae9cde234ffba32fbbf33d338
diff --git a/libc/bionic/pthread_internal.cpp b/libc/bionic/pthread_internal.cpp
index afafad8..1967ccf 100644
--- a/libc/bionic/pthread_internal.cpp
+++ b/libc/bionic/pthread_internal.cpp
@@ -81,16 +81,12 @@
 
 pthread_internal_t* __pthread_internal_find(pthread_t thread_id) {
   pthread_internal_t* thread = reinterpret_cast<pthread_internal_t*>(thread_id);
-  {
-    ScopedPthreadMutexLocker locker(&g_thread_list_lock);
+  ScopedPthreadMutexLocker locker(&g_thread_list_lock);
 
-    for (pthread_internal_t* t = g_thread_list; t != NULL; t = t->next) {
-      if (t == thread) {
-        return thread;
-      }
+  for (pthread_internal_t* t = g_thread_list; t != NULL; t = t->next) {
+    if (t == thread) {
+      return thread;
     }
   }
-  // TODO: Remove g_thread_list/g_thread_list_lock when the fatal error below doesn't happen.
-  __libc_fatal("No such thread: %p", thread);
   return NULL;
 }