Revert MySQL commit that disables writing on Windows while flush is in progress

Reason : after running sysbench tests (rw,update_no_index), in all cases
the throughout was considerably (approx 15%) better with the patch removed.

Reverted commit info
commit 8dc03bee3ade2edcc53a3a257346f4a0a9f0b44c
Author: Yasufumi Kinoshita <yasufumi.kinoshita@oracle.com>
Date:   Wed Nov 20 17:02:24 2013 +0900

Bug #17824101 : WL#7050 CAUSES RW PERFORMANCE REGRESSION AT SOME WINDOWS ENVIRONMENT

storage/innobase/log/log0log.cc

@@ -1280,16 +1280,6 @@ log_write_up_to(
 		return;
 	}
 
-#ifdef _WIN32
-	/* write requests during fil_flush() might not be good for Windows */
-	if (log_sys->n_pending_flushes > 0
-	    || !os_event_is_set(log_sys->flush_event)) {
-		log_write_mutex_exit();
-		os_event_wait(log_sys->flush_event);
-		goto loop;
-	}
-#endif /* _WIN32 */
-
 	/* If it is a write call we should just go ahead and do it
 	as we checked that write_lsn is not where we'd like it to
 	be. If we have to flush as well then we check if there is a