perf(WG): drop per-kill assist iteration; credit killer only

[Nyeriah]

Summary

Follow-up to #149. That PR fixed the actual bug (Slay Them All not crediting outside an active war) by crediting the killer directly and iterating nearby same-team players in the zone for assist credit. The iteration scales with war population (~100 players in a full WG) and runs on every kill — too costly for a daily-quest credit path.

• Drop the per-kill ForEachPlayerInZone scan.
• Keep fix(WG): Credit Slay Them All on any WG-zone kill, war or not #149's actual fix: credit the killer directly so the daily advances on any WG-zone kill, war or not, with both PvP-kill credit NPCs so crossfaction quests advance.
• Make lieutenant dedup O(1): single hash-set count(killer->GetGUID()) instead of an iteration.
• Core's existing lieutenant-only assist path in BattlefieldWG::HandleKill still fires unchanged for nearby allies on lieutenant kills.

Net result: same user-visible behavior as #149 for the killer (the only player that must be credited for the daily to advance), at O(1) per kill instead of O(zone-population).

Test plan

• Native Horde killer + native Alliance victim, outside active war → Horde "Slay Them All" (13180) counter ticks.
• Native Horde killer + flipped (real-Horde-to-Alliance) victim, outside war → counter ticks.
• Both above during active war → still tick.
• Crossfaction killer (real-Horde flipped to Alliance) killing a Horde victim → Horde quest counter ticks (via otherCredit).
• Lieutenant victim during war → killer credited exactly once (no double-count with core's lieutenant path).

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