BFT Block Puller: Remove redundant internal map in deliveryServiceImpl.blockproviders

[tock-ibm]

The gossip service:

• It contains a map of channelID -> DeliverService (an interface).
• The DeliveryService is implemented by a deliveryServiceImpl, which contains blockProviders a map of channelID -> Deliverer.
• The implementation shows that the external and internal maps are always keyed together, with the same channelID, which means that the internal map has always a single entry...

Remove the redundant internal map.

[tock-ibm]

Done

[tock-ibm]

There is a bug:

After the delivery service StopDeliveryForChannel is called, it should be possible to call StartDeliveryForChannel again.
After the delivery service Stop is called, it should not be possible to call <Start/Stop>DeliveryForChannel again.

This was introduced by #4260