We don't use reverse listing in any of our code, outside of tests, and
it is only exposed through libuplink in the
lib/uplink.(*Project).ListBuckets() API. We also don't know of any users
who might have a need for reverse listing through ListBuckets().
Since one of our prospective pointerdb backends can not support
backwards iteration, and because of the above considerations, we are
going to remove the reverse listing feature.
Change-Id: I8d2a1f33d01ee70b79918d584b8c671f57eef2a0
