Scripts/Quests: Fix Red Snapper - Very Tasty! GOb

Fix 'School of Red Snapper' now disappear correctly when using 'Draenei Fishing Net', the bug is caused because the GameObject is SpellFocus type.

Fix: #1673
look at this: https://gist.github.com/joschiwald/f3aa75306f5e36622058
i think its a better solution for this checkcast