Remove some dead ARC code. (#14340)

Every ARC buffer holds a reference on the header. It means headers with
buffers are never evictable. When we are evicting a header, there can
be no more buffers to free. Just assert that.

b_evict_lock seems not protecting anything now. Remove it.

Buffers checksum should also be freed with the last uncompressed buffer,
so it should not be there also when we are evicting the header.

Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.