Server/Database: Reduce differences between 3.3.5 and 6.x branches

Code functionality shouldn't have been modified.

src/server/database/Database/MySQLConnection.cpp

@@ -22,11 +22,9 @@
   #include <winsock2.h>
 #endif
 #include <mysql.h>
-#include <mysqld_error.h>
 #include <errmsg.h>
 
 #include "MySQLConnection.h"
-#include "MySQLThreading.h"
 #include "QueryResult.h"
 #include "SQLOperation.h"
 #include "PreparedStatement.h"