Remove libbacktrace definition of gettid.

Also simplifies the Darwin implementation of gettid, because
apparently libbacktrace had a better way of doing it.

Bug: 19517541

(cherry picked from commit 23f750b068ddf5b20fb5d153a798b22562368992)

Change-Id: I2f888e8ed7a2f5719973786cbcbb399a81867ee9
diff --git a/libbacktrace/thread_utils.c b/libbacktrace/thread_utils.c
index 6f4cd3c..e75f56e 100644
--- a/libbacktrace/thread_utils.c
+++ b/libbacktrace/thread_utils.c
@@ -16,25 +16,12 @@
 
 #include "thread_utils.h"
 
-#if defined(__APPLE__)
+#if !defined(__BIONIC__)
 
-#include <sys/syscall.h>
-
-// Mac OS >= 10.6 has a system call equivalent to Linux's gettid().
-pid_t gettid() {
-  return syscall(SYS_thread_selfid);
-}
-
-#elif !defined(__BIONIC__)
-
-// glibc doesn't implement or export either gettid or tgkill.
+// glibc doesn't implement or export tgkill.
 #include <unistd.h>
 #include <sys/syscall.h>
 
-pid_t gettid() {
-  return syscall(__NR_gettid);
-}
-
 int tgkill(int tgid, int tid, int sig) {
   return syscall(__NR_tgkill, tgid, tid, sig);
 }