tsan: remove stale comment

[dvyukov]

Summary

We do query it every 100ms now.
(GetRSS was fixed to not be dead slow IIRC)

Depends on D110152.

[dvyukov]

rebased and addressed comments

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

for (int i = 0;
     // Flush memory if requested.
     if (flags()->flush_memory_ms > 0) {
       if (last_flush + flags()->flush_memory_ms * kMs2Ns < now) {
         VPrintf(1, "ThreadSanitizer: periodic memory flush\n");
         FlushShadowMemory();
         last_flush = NanoTime();
       }
     }
-    // GetRSS can be expensive on huge programs, so don't do it every 100ms.
     if (flags()->memory_limit_mb > 0) {
       uptr rss = GetRSS();
       uptr limit = uptr(flags()->memory_limit_mb) << 20;
       VPrintf(1, "ThreadSanitizer: memory flush check"
                  " RSS=%llu LAST=%llu LIMIT=%llu\n",
               (u64)rss >> 20, (u64)last_rss >> 20, (u64)limit >> 20);
       if (2 * rss > limit + last_rss) {
         VPrintf(1, "ThreadSanitizer: flushing memory due to RSS\n");