cleanup fmt::format for std::format

Update the code to use std::format since gcc13 supports it now. Also
cleaned up some unused headers.

Change-Id: Ief2ad4df9e8b8227168f4f98213da8b3978fda03
Signed-off-by: Willy Tu <wltu@google.com>
diff --git a/subprojects/nemora-postd/src/host_manager.cpp b/subprojects/nemora-postd/src/host_manager.cpp
index eb0a2b9..b771146 100644
--- a/subprojects/nemora-postd/src/host_manager.cpp
+++ b/subprojects/nemora-postd/src/host_manager.cpp
@@ -14,17 +14,14 @@
 
 #include "host_manager.hpp"
 
-#include <fmt/format.h>
-
 #include <phosphor-logging/log.hpp>
 #include <sdbusplus/bus.hpp>
 #include <sdbusplus/message.hpp>
 
+#include <format>
 #include <functional>
-#include <iostream>
 #include <variant>
 
-using fmt::format;
 using phosphor::logging::level;
 using phosphor::logging::log;
 
@@ -79,7 +76,7 @@
     auto count = postcodes_.size();
     if (count > 0)
     {
-        std::string msg = format("Draining Postcodes. Count: {}.", count);
+        std::string msg = std::format("Draining Postcodes. Count: {}.", count);
         log<level::ERR>(msg.c_str());
     }