Making PLFA compatible with agda-2.8.0 and stdlib-2.3.0 (#1122)

Removed rewrite that did nothing as Agda now warns about that. That was
it, everything else worked as is.

Author: JacquesCarette

src/plfa/part3/Soundness.lagda.md

@@ -394,7 +394,7 @@ subst-reflect-var {Γ}{Δ}{γ}{x}{v}{σ} xv
   const-env-ok : γ `⊢ σ ↓ const-env x v
   const-env-ok y with x var≟ y
   ... | yes x≡y rewrite sym x≡y | same-const-env {Γ}{x}{v} = xv
-  ... | no x≢y rewrite diff-const-env {Γ}{x}{y}{v} x≢y = ⊥-intro
+  ... | no x≢y = ⊥-intro
 ```