TestPGLog::merge_old_entry: ne.version cannot be oe.version

Otherwise, it would not be divergent!

Signed-off-by: Samuel Just <sam.just@inktank.com>
another unnecessary remove_snaps assert
TestPGLog::merge_old_entry: we no longer use merge_old_entry this way

This needs to be replaced with an equivalent test of
_merge_object_divergent_entries.

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