Core/Spell: Fix farsight log error "Player requests non-existing seer"

Partially revert http://github.com/TrinityCore/TrinityCore/commit/167ca565d044915f01ead4b189754ab1c44cc981 which wrongly used TYPEMASK_UNIT to denote Creatures and Players, so when a Player sent farsight opcode with another Player as target HandleFarSightOpcode() couldn't find the right Unit filtering by typemask TYPEMASK_SEER.