chore: remove theorem

I moved the theorem to another pull request:
leanprover-community#994.

Batteries/Data/List/Lemmas.lean

@@ -500,25 +500,6 @@ theorem ne_nil_of_not_prefix (h : ¬l₁ <+: l₂) : l₁ ≠ [] := by
   intro heq
   simp [heq, nil_prefix] at h
 
-theorem not_prefix_and_not_prefix_symm_iff_exists [BEq α] [LawfulBEq α] [DecidableEq α]
-    {l₁ l₂ : List α} : ¬l₁ <+: l₂ ∧ ¬l₂ <+: l₁ ↔ ∃ c₁ c₂ pre suf₁ suf₂, c₁ ≠ c₂ ∧
-      l₁ = pre ++ c₁ :: suf₁ ∧ l₂ = pre ++ c₂ :: suf₂ := by
-  constructor <;> intro h
-  · obtain ⟨c₁, l₁, rfl⟩ := l₁.exists_cons_of_ne_nil (ne_nil_of_not_prefix h.1)
-    obtain ⟨c₂, l₂, rfl⟩ := l₂.exists_cons_of_ne_nil (ne_nil_of_not_prefix h.2)
-    simp only [cons_prefix_cons, not_and] at h
-    cases Decidable.em (c₁ = c₂)
-    · subst c₂
-      simp only [forall_const] at h
-      let ⟨c₁', c₂', pre, suf₁, suf₂, hc, heq₁, heq₂⟩ :=
-        not_prefix_and_not_prefix_symm_iff_exists.mp h
-      exact ⟨c₁', c₂', c₁::pre, suf₁, suf₂, hc, by simp [heq₁], by simp [heq₂]⟩
-    · next hc =>
-      exact ⟨c₁, c₂, [], l₁, l₂, hc, nil_append .., nil_append ..⟩
-  · let ⟨c₁, c₂, pre, suf₁, suf₂, hc, heq₁, heq₂⟩ := h
-    rw [heq₁, heq₂]
-    simp [prefix_append_right_inj, cons_prefix_cons, hc, hc.symm]
-
 /-! ### drop -/
 
 theorem disjoint_take_drop : ∀ {l : List α}, l.Nodup → m ≤ n → Disjoint (l.take m) (l.drop n)