nfsd4: remove unneeded CLAIM_DELEGATE_CUR workaround

0c12eaff "nfsd: don't break lease on
CLAIM_DELEGATE_CUR" was a temporary workaround for a problem fixed
properly in the vfs layer by 778fc546
"locks: fix tracking of inprogress lease breaks", so we can revert that
change (but keeping some minor cleanup from that commit).

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