ReplicatedPG::_copy_some: don't check snaps, we handle it in finish_promote

For one thing, our requested snap may well have mapped on to head if
our target clone happened to be the most recent thing flushed.  We
have to use our own snapset to judge whether the clone still exists,
which we do in finish_promote.

Signed-off-by: Samuel Just <sam.just@inktank.com>