Simplification: remove the ability to contribute collateral at subnet creation

[raulk]

Currently, if value is sent when registering a new subnet, we record it as collateral contributed by the creator, who will now be enlisted as a validator in the network. While this might be useful in some situations, it can be achieved by performing both actions separately, in an atomic manner if needed, either through a Multicall contract or through a dedicated utility smart contract.

In other words, this feature is actually an aggregate of two other features, and therefore my view is that it can be dropped from the base protocol/mechanics in order to reduce complexity. As far as I know, it is not being actively used by any adopter at the current time. And this work #1130 was forced to add extra complexity to sustain this feature as we added a new collateral asset kind.

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We can apply the same simplification to circulating supply -- I would just drop this as a subnet creation parameter.

[cryptoAtwill]

@raulk are you referring to this method?

When the subnet is registered, it does not forward the collateral? Or actually break it into two separate calls?

[raulk]

Gathering user input in Slack: https://filecoinproject.slack.com/archives/C04U4GPK3QR/p1727254644064279

[raulk]

@cryptoAtwill I'm referring to the whole flow, which includes that method, but also stuff in LibSubnetActor and other places.