Delete unused code and moved some lemmas into Lattice.lean

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lean/Spa/Lattice/IterProd.lean

@@ -13,15 +13,6 @@ namespace IterProd
 
 variable {A B : Type u}
 
-instance decidableEq [DecidableEq A] [DecidableEq B] :
-    ∀ k, DecidableEq (IterProd A B k)
-  | 0 => inferInstanceAs (DecidableEq B)
-  | k + 1 => @instDecidableEqProd A (IterProd A B k) _ (decidableEq k)
-
-def build (a : A) (b : B) : (k : ℕ) → IterProd A B k
-  | 0 => b
-  | k + 1 => (a, build a b k)
-
 def fixedHeight [FiniteHeightLattice A] [FiniteHeightLattice B] :
     ∀ k, FiniteHeightLattice (IterProd A B k)
   | 0 => inferInstanceAs (FiniteHeightLattice B)