Remove dead rb_ractor_t in ractor_selector

Author: peterzhu2118

ractor_sync.c

@@ -1221,7 +1221,6 @@ ractor_try_send(rb_execution_context_t *ec, const struct ractor_port *rp, VALUE
 // Ractor::Selector
 
 struct ractor_selector {
-    rb_ractor_t *r;
     struct st_table *ports; // rpv -> rp
 
 };