DB/Vendor: link some Arena vendors to the season

Closes #21749

sql/updates/world/3.3.5/2018_04_04_08_world_335.sql

@@ -0,0 +1,8 @@
+-- 
+DELETE FROM `game_event_creature` WHERE (`eventEntry`=55 AND `guid`=70996) OR (`eventEntry`=56 AND `guid` IN (75914, 79018, 75475, 88156));
+INSERT INTO `game_event_creature` (`eventEntry`, `guid`) VALUES
+(55, 70996),
+(56, 75914),
+(56, 79018),
+(56, 75475),
+(56, 88156);