evolve: remove the unnecessary condition to check empty successors set
I annotated the history of this "newer == [()]" condition and found
that it was added in the initial stages of evolution project (more than
8 years ago) and there is no test where we get [()] as a successor set.
So looks like "if not newer" is enough to check if it is empty.
I also looked into obsutil.successorssets() implementation and
I don't think it would give us [()] in any case.
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