Disable peer.deliveryclient.blockGossipEnabled in sample core.yaml

[dvusboy]

In the release note of v2.5.0, under "FAB-15317: Block dissemination via gossip is deprecated", the recommended setting for blockGossipEnabled is false. I think sampleconfig/core.yaml should reflect that.

[denyeart]

Moving to Fabric v3.0... in Fabric v3.0 either this default will be changed or the gossip block dissemination function will be entirely removed... see proposal in hyperledger/fabric-rfcs#49.

[hxx258456]

Will newly joined nodes be affected when they perform block synchronization?