Remove contract_id() in favor of ContractId::this()

[bitzoic]

Description

We currently support 2 methods of getting the contract id of a contract in an internal context:

• contract_id()
• ContractId::this()

The this() associated function is a constructor that returns Self and should be the primary method of getting the contract id. This idiomatically follows Rust's syntax and contract_id() has been removed.

The same syntax is followed with AssetId::base().

Closes #5834

Checklist

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