message: Update CHANGELOG for demo-staking removal
diff: - @cosmjs/cosmwasm: Rename `CosmWasmClient.postTx` method to `.broadcastTx`.
  - @cosmjs/cosmwasm: Rename `SigningCosmWasmClient.signAndPost` method to
  `.signAndBroadcast`.
  +- @cosmjs/demo-staking: Remove package and supporting scripts.
  - @cosmjs/launchpad: Rename `CosmosClient.postTx` method to `.broadcastTx`.
  - @cosmjs/launchpad: Rename `SigningCosmosClient.signAndPost` method to
  `.signAndBroadcast`.
  