Remove duplicate struct declaration

internal/vm.h

@@ -96,7 +96,6 @@ struct rb_iseq_struct;
 const struct rb_callcache *rb_vm_search_method_slowpath(const struct rb_callinfo *ci, VALUE klass);
 
 /* vm_method.c */
-struct rb_execution_context_struct;
 int rb_ec_obj_respond_to(struct rb_execution_context_struct *ec, VALUE obj, ID id, int priv);
 
 void rb_clear_constant_cache(void);