rpc-v2/tx: Remove the broadcast event from `transaction_submitAndWatch` (#3321)

This PR backports the changes from the rpc-v2 spec:
https://github.com/paritytech/json-rpc-interface-spec/pull/134

The `Broadcasted` event has been removed:
- it is hard to enforce a `Dropped { broadcasted: bool }` event in cases
of a load-balancer being placed in front of an RPC server
- when the server exists, it is impossible to guarantee this field if
the server did not previously send a `Broadcasted` event
- the number of peers reported by this event does not guarantee that
peers are unique
- the same peer can disconnect and reconnect, increasing this metric
number
- the number of peers that receive this transaction offers no guarantee
about the transaction being included in the chain at a later time


cc @paritytech/subxt-team

---------

Signed-off-by: Alexandru Vasile <[email protected]>
Co-authored-by: James Wilson <[email protected]>