Moved should_flush_ to header

src/logger.cpp

@@ -81,10 +81,4 @@ void logger::flush_() {
         }
     }
 }
-
-bool logger::should_flush_(const details::log_msg &msg) const {
-    const auto flush_level = flush_level_.load(std::memory_order_relaxed);
-    return (msg.log_level >= flush_level) && (msg.log_level != level::off);
-}
-
 }  // namespace spdlog