sysctl.3: remove some confusing text about NFS_NIOTHREADS

sysctl.8: remove the vfs.nfs.iothreads example, since a) showing how
to set a default value is not particularly helpful and b) EXAMPLES is
bloated enough as it is; further, remove the text which points to other pages
for further discussion, since those pages no longer provide relevant info

diff started by an email from Jan Stary; blambert helped me prepare this diff

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