glsl: Eliminate ir_assignment::condition

Reformatting is left for the next commit.

v2: Remove assignments from the contructors. :face_palm:

Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14573>

src/compiler/glsl/ir.h

@@ -1518,23 +1518,6 @@ public:
     */
    ir_rvalue *rhs;
 
-   /**
-    * Optional condition for the assignment.
-    */
-private:
-   ir_rvalue *unused_condition;
-
-public:
-   inline ir_rvalue *get_condition()
-   {
-      return unused_condition;
-   }
-
-   inline const ir_rvalue *get_condition() const
-   {
-      return unused_condition;
-   }
-
    /**
     * Component mask written
     *