[ast] Remove IsCast

This was missed in the general removal, cleanup.

Change-Id: I7912bb86e73cd9e38d497fab5995f7740a9dfbc6
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/29000
Commit-Queue: David Neto <dneto@google.com>
Reviewed-by: David Neto <dneto@google.com>
diff --git a/src/ast/expression.cc b/src/ast/expression.cc
index f58c138..6a14603 100644
--- a/src/ast/expression.cc
+++ b/src/ast/expression.cc
@@ -52,10 +52,6 @@
   return false;
 }
 
-bool Expression::IsCast() const {
-  return false;
-}
-
 bool Expression::IsIdentifier() const {
   return false;
 }
diff --git a/src/ast/expression.h b/src/ast/expression.h
index dc8ce58..be2c0f6 100644
--- a/src/ast/expression.h
+++ b/src/ast/expression.h
@@ -50,8 +50,6 @@
   virtual bool IsBitcast() const;
   /// @returns true if this is a call expression
   virtual bool IsCall() const;
-  /// @returns true if this is a cast expression
-  virtual bool IsCast() const;
   /// @returns true if this is an identifier expression
   virtual bool IsIdentifier() const;
   /// @returns true if this is an constructor expression