tsan: remove unused variable

[dvyukov]

Summary

Depends on D113983.

[dvyukov]

rebase

compiler-rt/lib/tsan/rtl/tsan_rtl.cpp

static void *BackgroundThread(void *arg) {
   // enabled even when the thread function exits (e.g. during pthread thread
   // shutdown code).
   cur_thread_init()->ignore_interceptors++;
   const u64 kMs2Ns = 1000 * 1000;
   const u64 start = NanoTime();
 
   u64 last_flush = start;
   uptr last_rss = 0;
-  for (int i = 0;
-      atomic_load(&ctx->stop_background_thread, memory_order_relaxed) == 0;
-      i++) {
+  while (!atomic_load_relaxed(&ctx->stop_background_thread)) {
     SleepForMillis(100);
     u64 now = NanoTime();
 
     // Flush memory if requested.
     if (flags()->flush_memory_ms > 0) {
       if (last_flush + flags()->flush_memory_ms * kMs2Ns < now) {
         VReport(1, "ThreadSanitizer: periodic memory flush\n");
         FlushShadowMemory();