DB/NPC: Use the same flags_extra (no skill gain) and immunities (grip and bleed) to all the training dummies.

Closes #1853

sql/updates/world/2012_05_26_02_world_creature_template.sql

@@ -0,0 +1,5 @@
+-- Training dummies
+UPDATE `creature_template` SET
+    `flags_extra`=0x40000, /* NO_SKILLGAIN */
+    `mechanic_immune_mask`=0x20|0x4000 /* GRIP|BLEED */ /* two of the templates already had bleed immunity, is that correct? */
+WHERE `ScriptName`='npc_training_dummy';