Deprecated and now unused method lastBlockWithDividend()
This method was suspected to be orphan, but it was actually used by Duniter UI (to display units using UD).
However since LevelDB migration of Duniter 1.7, this method has become super slow and resource consuming.
I updated DuniterUI to use another, way more performant method.
So this lastBlockWithDividend()
can be removed.