Simplify clang workarounds.

tpool/tpool_generic.cc

@@ -561,15 +561,8 @@ void thread_pool_generic::worker_main(worker_data *thread_var)
   Helper function, to be used inside maintenance callback,
   before m_last_activity is updated
 */
-#ifndef __has_feature
-# define __has_feature(x) 0
-#endif
-#if __has_feature(memory_sanitizer)
-const /* WITH_MSAN in clang++-11 does not work with constexpr */
-#else
-constexpr
-#endif
-auto invalid_timestamp=  std::chrono::system_clock::time_point::max();
+
+static const auto invalid_timestamp=  std::chrono::system_clock::time_point::max();
 constexpr auto max_idle_time= std::chrono::minutes(1);
 
 /* Time since maintenance timer had nothing to do */