nfsd: remove nfs4_reset_lease() declarations

The function was removed a long time ago, but the declaration
and a dummy implementation are still there, referencing the
deprecated time_t type.

Remove both.

Fixes: f958a132 ("nfsd4: remove unnecessary lease-setting function")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>