TestPGLog:rewind_divergent_log: set prior_version for delete

Signed-off-by: Samuel Just <sam.just@inktank.com>
TestPGLog:rewind_divergent_log: don't remove_snap be empty if divergent delete

It's harmless to do the delete, and it avoids another branch.

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