nfsd4: remove unneeded CLAIM_DELEGATE_CUR workaround

0c12eaffdf09466f36a9ffe970dda8f4aeb6efc0 "nfsd: don't break lease on
CLAIM_DELEGATE_CUR" was a temporary workaround for a problem fixed
properly in the vfs layer by 778fc546f749c588aa2f6cd50215d2715c374252
"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>