*: simplify pointCollapsingIter, use vanilla iters for foreign SSTs

Previously we were using pointCollapsingIters to collapse obsolete
points in foreign sstables. This was necessary because of snapshots
that were open at the time those sstables were written, however they
really blew up code complexity and performance. This change
transitions to using vanilla sstable iterators for foreign sstables
with `hideObsoletePoints` set to true. We error out if the
sstable format for a foreign sstable is not high enough to support
the obsolete flag.

Also remove all cases in the pointCollapsingIter that were not
used by ScanInternal (which is the last remaining user of it).

Part of cockroachdb#2756.