`zetacored` : add necessary messages to reduce the observer set on a live network

[kingpinXD]

Updating the observer-signer set on a live network includes

• Adjusting observer set
• Adjusting the node account list
• TSS keygen ceremony with the new list

We currently have the necessary messages required to generate a new TSS with an observer added to this set

• types.MsgAddObserver
  https://github.com/zeta-chain/zeta-node/blob/737e30d66c3538255e943c752d0f1bf67f4f9fc9/x/observer/keeper/msg_server_add_observer.go#L1-L71

However we do not have a similar message to reduce the number of observer-signers , This needs to be implemeted to do a Keygen with a reduced observer set

Related : #3998