ReplicatedPG: remove long-dead branch

This was confusing the heck out of me when trying to figure out
why I was hitting an assert.

Signed-off-by: Greg Farnum <greg@inktank.com>
PG: update the child's last_user_version when splitting

If we don't fill this in then everything breaks following a split.

Signed-off-by: Greg Farnum <greg@inktank.com>