Core/QuestPOI: Mitigate possible DoS with CMSG_QUEST_POI_QUERY

Avoid sending POIs for same quest if the client somehow sent duplicates quest id in same CMSG_QUEST_POI_QUERY packet. This also reduce the effects of possible DoS and increases the difficulty to cause it.
Fix a typo which caused no quest POIs to be sent at all if the client queried data for 25 quests.
Core/Misc: Code cleanup

Remove old unused opcode handler WorldSession::HandlePageQuerySkippedOpcode() for CMSG_PAGE_TEXT_QUERY since it's already handled by WorldSession::HandlePageTextQueryOpcode()
Core/Misc: Added utility method for checking is two sorted containers have a common element
Scripts: Remove unused variables (kept as comment)
Script/Shadowmoon Valley: Remove hack in npc_shadowmoon_tuber_node.

This script will be converted to SAI (if possible) later.