[libcxx testing] Remove ALLOW_RETRIES from sleep_for.pass.cpp

Operating systems are best effort by default, so we cannot assume that
sleep-like APIs return as soon as we'd like.

Even if a sleep-like API returns when we want it to, the potential for
preemption means that attempts to measure time are subject to delays.

libcxx/test/libcxx/thread/thread.threads/thread.thread.this/sleep_for.pass.cpp

@@ -8,8 +8,6 @@
 //
 // UNSUPPORTED: libcpp-has-no-threads
 
-// ALLOW_RETRIES: 2
-
 // This test uses the POSIX header <sys/time.h> which Windows doesn't provide
 // UNSUPPORTED: windows
 
@@ -62,10 +60,9 @@ int main(int, char**)
   time_point t0 = Clock::now();
   std::this_thread::sleep_for(ms);
   time_point t1 = Clock::now();
-    std::chrono::nanoseconds ns = (t1 - t0) - ms;
-    std::chrono::nanoseconds err = 5 * ms / 100;
-    // The time slept is within 5% of 500ms
-    assert(std::abs(ns.count()) < err.count());
+  // NOTE: Operating systems are (by default) best effort and therefore we may
+  // have slept longer, perhaps much longer than we requested.
+  assert(t1 - t0 >= ms);
 
   return 0;
 }