ir_reader: Remove s_list::length() method.

Most code now relies on the pattern matcher rather than this function,
and for the only remaining case, not using this saves an iteration.

src/glsl/s_expression.h

@@ -139,7 +139,6 @@ public:
    s_list();
 
    virtual bool is_list() const { return true; }
-   unsigned length() const;
 
    void print();