fix: spires no longer leave walls

Leaving walls is OK for statues (whose placement is highly controlled),
but bad for spires. Reason: I just encountered a spire in a doorway,
upon killing it to enter the room, the corpse swapped with me, meaning I
was standing in a wall before moving out.

In the future spires will leave heaps of tangled metal for cleaners to
remove (and replace? hmm).

src/mobs.zig

@@ -778,7 +778,7 @@ fn createSpireTemplate(
             .memory_duration = 77,
             .life_type = .Construct,
             .blood = null,
-            .corpse = .Wall,
+            .corpse = .None,
             .immobile = true,
             .innate_resists = .{ .rFume = 100, .rFire = 25, .rElec = 25, .Armor = 20 },
             .stats = .{ .Willpower = opts.willpower, .Evade = 0, .Vision = 8 },