Remove Existing Class Is_true.

Having Is_true as a type class caused problems with rewrite: when the
rewrited lemma has a premise of the shape Is_true, the rewrite tactic
will complain that it cannot find a type class instance, instead
of generating a goal for that premise.

prelude/base.v

@@ -375,8 +375,6 @@ Notation zip := (zip_with pair).
 Coercion Is_true : bool >-> Sortclass.
 Hint Unfold Is_true.
 Hint Immediate Is_true_eq_left.
-Existing Class Is_true.
-Instance true_Is_true : Is_true true := I.
 Hint Resolve orb_prop_intro andb_prop_intro.
 Notation "(&&)" := andb (only parsing).
 Notation "(||)" := orb (only parsing).