Remove `maturity` field from the `Coin`

[xgreenx]

When we create a Coin, we set maturity to zero because the user doesn't have a way to specify the maturity for the Output. Because of that, it is a useless field right now.

Either we need to provide a way for the user to set it or remove it completely.

As a fast solution, we can remove it for now and add support for it later with the upgrade.

[Voxelot]

@xgreenx should this issue be closed as well? Also we should update the specs.

[xgreenx]

It is not implemented yet, so we don't need to close it, we closed the duplicate=) Yeah, we need to update the specification as well not to include maturity in the Coin input.

[Dentosal]

Mostly implemented in https://github.com/FuelLabs/fuel-vm/pull/684, although we still need a follow-up for fuel-core after that.

[xgreenx]

Finished with https://github.com/FuelLabs/fuel-core/pull/1714