ast: Remove ReturnStatement set_value()

Unused.

Bug: tint:390
Change-Id: If2832da7181e168f8a3bb8c986d8ee2d469b25b2
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/35011
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: dan sinclair <dsinclair@chromium.org>

src/ast/return_statement.h

@@ -38,9 +38,6 @@ class ReturnStatement : public Castable<ReturnStatement, Statement> {
   ReturnStatement(ReturnStatement&&);
   ~ReturnStatement() override;
 
-  /// Sets the value
-  /// @param value the value
-  void set_value(Expression* value) { value_ = value; }
   /// @returns the value
   Expression* value() const { return value_; }
   /// @returns true if the return has a value