fix(data/finset): remove partial_order instance

* The partial_order instance is subsumed by the lattice instance.
* The other two iff.rfl theorems do not seem necessary.

data/finset.lean

@@ -75,16 +75,6 @@ theorem subset_iff {s₁ s₂ : finset α} : s₁ ⊆ s₂ ↔ ∀ ⦃x⦄, x 
 
 instance : has_ssubset (finset α) := ⟨λa b, a ⊆ b ∧ ¬ b ⊆ a⟩
 
-instance : partial_order (finset α) :=
-{ le := (⊆),
-  lt := (⊂),
-  le_refl := subset.refl,
-  le_trans := @subset.trans _,
-  le_antisymm := @subset.antisymm _ }
-
-@[simp] theorem le_iff_subset {s₁ s₂ : finset α} : s₁ ≤ s₂ ↔ s₁ ⊆ s₂ := iff.rfl
-@[simp] theorem lt_iff_ssubset {s₁ s₂ : finset α} : s₁ < s₂ ↔ s₁ ⊂ s₂ := iff.rfl
-
 @[simp] theorem val_lt_iff {s₁ s₂ : finset α} : s₁.1 < s₂.1 ↔ s₁ ⊂ s₂ :=
 and_congr val_le_iff $ not_congr val_le_iff