Documentation: remove overloads-avoided counter from knfsd-stats.txt

The 'overloads-avoided' counter itself was removed several years ago by
commit 78c210ef

 (Revert "knfsd: avoid overloading the CPU scheduler with
enormous load averages").

Signed-off-by: Scott Mayhew <smayhew@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>