perf(CategoryTheory/Triangulated/TriangleShift): replace cat_disch by faster tactic (#41106)

Replace `cat_disch` with `intros; ext <;> simp` in `Triangle.shiftFunctorAdd'`. See [Zulip](https://leanprover.zulipchat.com/#narrow/channel/116290-rss/topic/Significant.20commits.20to.20mathlib4/near/606901493).

Author: plp127

Mathlib/CategoryTheory/Triangulated/TriangleShift.lean

@@ -111,7 +111,7 @@ noncomputable def Triangle.shiftFunctorAdd' (a b n : ℤ) (h : a + b = n) :
           ← dsimp% (CategoryTheory.shiftFunctorAdd' C a b (a + b) rfl).hom.naturality_assoc]
         simp only [shiftFunctorAdd'_eq_shiftFunctorAdd, Int.negOnePow_add,
           shiftFunctorComm_hom_app_comp_shift_shiftFunctorAdd_hom_app, add_comm a]))
-    (by cat_disch)
+    (by intros; ext <;> simp)
 
 set_option backward.defeqAttrib.useBackward true in
 set_option backward.isDefEq.respectTransparency false in