glsl: Don't dead-built-in varying eliminate in the condition of an assignment

At this point, this should always be NULL.

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

src/compiler/glsl/opt_dead_builtin_varyings.cpp

@@ -489,7 +489,6 @@ public:
    virtual ir_visitor_status visit_leave(ir_assignment *ir)
    {
       handle_rvalue(&ir->rhs);
-      handle_rvalue(&ir->condition);
 
       /* We have to use set_lhs when changing the LHS of an assignment. */
       ir_rvalue *lhs = ir->lhs;