Remove dangerous "remove warning" hack, cast to long

will truncate the time value on systems where "long" are 32bit,
instead use time_t as datatype

sql/log.cc

@@ -1784,7 +1784,7 @@ void MYSQL_LOG::rotate_and_purge(uint flags)
 #ifdef HAVE_REPLICATION
     if (expire_logs_days)
     {
-      long purge_time= (long) (time(0) - expire_logs_days*24*60*60);
+      time_t purge_time= time(0) - expire_logs_days*24*60*60;
       if (purge_time >= 0)
         purge_logs_before_date(purge_time);
     }

sql/mysqld.cc

@@ -3271,7 +3271,7 @@ server.");
 #ifdef HAVE_REPLICATION
   if (opt_bin_log && expire_logs_days)
   {
-    long purge_time= (long) (time(0) - expire_logs_days*24*60*60);
+    time_t purge_time= time(0) - expire_logs_days*24*60*60;
     if (purge_time >= 0)
       mysql_bin_log.purge_logs_before_date(purge_time);
   }