refactor(contracts): refactor contracts code

[ctrlc03]

Description

Refactor by removing dead code, and various optimizations.

• Remove dead code
• Remove VkRegistry from ExtContracts and pass it directly to Mp, Tally and Subsidy - Poll does not need it and we save on external calls on these contracts
• Remove onlyOwner from pollFactory to save on gas - It's fine if anyone deploys a Poll using the PollFactory - only Polls that have access to privileged MACI's functions are Polls deployed via MACI's deployPoll function
• Remove init function from MACI's contract as this is not needed: If PollFactory is open to all, there is no need to check that ownership is assigned to the MACI contract, so we can remove the init function and modifiers to save gas, and just pass the top up credit address in the constructor.

Related issue(s)

Re #843

Confirmation

• I have read and understand MACI's contributor guidelines and code of conduct requirements.
• I have read and understand MACI's GitHub processes documentation.
• I have read and understand MACI's testing guide.

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[baumstern]

decentralizing PollFactory +1