Move tac_exist from classes to coq_tactics where it belongs.

proofmode/classes.v

@@ -301,10 +301,6 @@ Global Arguments from_exist {_} _ _ {_}.
 Global Instance from_exist_exist {A} (Φ: A → uPred M): FromExist (∃ a, Φ a) Φ.
 Proof. done. Qed.
 
-Lemma tac_exist {A} Δ P (Φ : A → uPred M) :
-  FromExist P Φ → (∃ a, Δ ⊢ Φ a) → Δ ⊢ P.
-Proof. intros ? [a ?]. rewrite -(from_exist P). eauto using exist_intro'. Qed.
-
 Class IntoExist {A} (P : uPred M) (Φ : A → uPred M) :=
   into_exist : P ⊢ ∃ x, Φ x.
 Global Arguments into_exist {_} _ _ {_}.

proofmode/coq_tactics.v

@@ -719,6 +719,10 @@ Lemma tac_forall_revert {A} Δ (Φ : A → uPred M) :
 Proof. intros HΔ a. by rewrite HΔ (forall_elim a). Qed.
 
 (** * Existential *)
+Lemma tac_exist {A} Δ P (Φ : A → uPred M) :
+  FromExist P Φ → (∃ a, Δ ⊢ Φ a) → Δ ⊢ P.
+Proof. intros ? [a ?]. rewrite -(from_exist P). eauto using exist_intro'. Qed.
+
 Lemma tac_exist_destruct {A} Δ i p j P (Φ : A → uPred M) Q :
   envs_lookup i Δ = Some (p, P) → IntoExist P Φ →
   (∀ a, ∃ Δ',