Remove block_connected de-duplication logic

Calling block_connected for the same block was necessary when clients
were expected to re-fetch filtered blocks with relevant transactions. At
the time, all listeners were notified of the connected block again for
re-scanning. Thus, de-duplication logic was put in place.

However, this requirement is no longer applicable for ChannelManager as
of commit bd39b20f642e042981e4fdd5f3600a357be51931, so remove this
logic.