Remove IsDecidable record in favor of a plain definition

Signed-off-by: Danila Fedorin <danila.fedorin@gmail.com>

Fixedpoint.agda

@@ -5,7 +5,7 @@ module Fixedpoint {a} {A : Set a}
                   {h : ℕ}
                   {_≈_ : A → A → Set a}
                   {_⊔_ : A → A → A} {_⊓_ : A → A → A}
-                  (decA : IsDecidable _≈_)
+                  (≈-dec : IsDecidable _≈_)
                   (flA : IsFiniteHeightLattice A h _≈_ _⊔_ _⊓_)
                   (f : A → A)
                   (Monotonicᶠ : Monotonic (IsFiniteHeightLattice._≼_ flA)
@@ -16,12 +16,11 @@ open import Data.Product using (_×_; Σ; _,_; proj₁; proj₂)
 open import Relation.Binary.PropositionalEquality using (_≡_; sym)
 open import Relation.Nullary using (Dec; ¬_; yes; no)
 
+open IsFiniteHeightLattice flA
+
 import Chain
 module ChainA = Chain _≈_ ≈-equiv _≺_ ≺-cong
 
-open IsDecidable decA using () renaming (R-dec to ≈-dec)
-open IsFiniteHeightLattice flA
-
 private
     ⊥ᴬ : A
     ⊥ᴬ = proj₁ (proj₁ (proj₁ fixedHeight))