chore(library/standard): remove workarounds

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

library/standard/data/list/basic.lean

@@ -221,9 +221,6 @@ list_induction_on s
 theorem mem_concat (x : T) (s t : list T) : x ∈ s ++ t ↔ x ∈ s ∨ x ∈ t
 := iff_intro (mem_concat_imp_or _ _ _) (mem_or_imp_concat _ _ _)
 
-section
-set_option unifier.expensive true -- TODO(Leo): remove after we add delta-split step
-#erase_cache mem_split
 theorem mem_split (x : T) (l : list T) : x ∈ l → ∃s t : list T, l = s ++ (x :: t) :=
 list_induction_on l
   (take H : x ∈ nil, false_elim _ (iff_elim_left (mem_nil x) H))
@@ -240,7 +237,6 @@ list_induction_on l
         have H4 : y :: l = (y :: s) ++ (x :: t),
           from subst H3 (refl (y :: l)),
         exists_intro _ (exists_intro _ H4)))
-end
 
 -- Find
 -- ----

library/standard/logic/axioms/examples/diaconescu.lean

@@ -32,7 +32,6 @@ or_elim u_def
     (assume Hp : p, or_inr Hp))
   (assume Hp : p, or_inr Hp)
 
-set_option unifier.expensive true
 lemma p_implies_uv [private] : p → u = v :=
 assume Hp : p,
   have Hpred : (λ x, x = true ∨ p) = (λ x, x = false ∨ p), from