Clean up silly code case.

This silly code segment has existed in the sources since it was brought
into FreeBSD 10 years ago. I honestly have no idea why this was done.
It was possible that I thought that it might have been better to not
set B_ASYNC for the "else" case, but I can't remember.
Anyhow, this patch gets rid of the if/else that does the same thing
either way, since it looks silly and upsets a static analyser.
This will have no semantic effect on the NFS client.

PR: 238167