log_heap_visible(): remove unused parameter and update comment

Hi,

While resuming the work on [1] I noticed that:

- there is an unused parameter in log_heap_visible()
- the comment associated to the function is not in "sync" with the 
current implementation (referring a "block" that is not involved anymore)

Attached a tiny patch as an attempt to address the above remarks.


[1]: https://commitfest.postgresql.org/39/3740/

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com