Remove wrong MECHANIC_SLOW_ATTACK immunity from creatures

- Bosses need to be affected by tank debuffs (ie Thunder Clap, Judgements of the Just, Infected Wounds, Frost Fever)

Ref 8ecde2f96b

sql/updates/world/3.3.5/2017_12_30_00_world.sql

@@ -0,0 +1 @@
+UPDATE `creature_template` SET `mechanic_immune_mask` = `mechanic_immune_mask` & 0xFFFFFF7F;