Unnecessary storage of on-chain service id in synchronized data

[Adamantios]

The on_chain_service_id does not need to be stored in the synced data, as it is already in the context of the agent and can be accessed via the params attribute, e.g. here:

trader/packages/valory/skills/trader_abci/handlers.py

Line 183 in 8018215

"service_id": self.staking_synchronized_data.service_id,

Originally posted by @Adamantios in #430 (comment)