drop removeBuilding, which nothing has called for a while

A public mutator that erased a building with no refund and no deconstruction
queue, documented as being "used for deaths". Nothing calls it: stations and the
HQ became ECS entities, so a death releases its tiles through
unregisterTileOccupancy instead, and every player-driven removal goes through
deconstruct.

Worth removing rather than leaving inert -- it was a way to take a building out
of the world that skipped the refund and the queue, sitting in public view on the
class whose whole job is that lifecycle.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ne3mejABZoLWKLh8fgpM3x

src/lib/sim/BuildingSystem.h

@@ -112,10 +112,6 @@ public:
                             const std::vector<std::string>& surfaceMask,
                             QPoint anchor, Rotation rotation);
 
-    // Remove an operational building by id without refund (used for deaths).
-    // Returns true if found and removed.
-    bool removeBuilding(FactoryState& state, BuildingId id);
-
     // Mutable iteration over all operational buildings.
     void forEachBuilding(FactoryState& state, std::function<void(Building&)> fn);