Commit b09ff536 left behind some outdated advice in the long_desc field
of the GUC "effective_io_concurrency".  Remove it.

Back-patch to 13.
Reported-by: Andrew Gierth <andrew@tao11.riddles.org.uk>
Reviewed-by: Julien Rouhaud <rjuju123@gmail.com>
Discussion: https://postgr.es/m/CA%2BhUKGJyyWqFBxL9gEj-qtjBThGjhAOBE8GBnF8MUJOJ3vrfag%40mail.gmail.com