Remove unused member from AnyConstructor.

This appears to be leftover from pre-Constructor-refactor times.

Change-Id: Id965468cf639fe7e55ca80f7f59bb8c84dbe12b1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/499380
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>

src/sksl/ir/SkSLConstructor.h

@@ -73,8 +73,6 @@ public:
     ComparisonResult compareConstant(const Expression& other) const override;
 
 private:
-    std::unique_ptr<Expression> fArgument;
-
     using INHERITED = Expression;
 };