Remove carbon_offset from structs

[jchen293]

Description

Removed CarbonOffset parameter from createShipmentRequest, buyShipmentRequest, and buyShipmentRequest structs as EasyPost now offers Carbon Neutral shipments by default for free

Testing

Re-recorded all cassettes(more than a year old) and updated unit tests

Pull Request Type

Please select the option(s) that are relevant to this PR.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement (fixing a typo, updating readme, renaming a variable name, etc.)

[nwithan8]

The nice thing about Go's inability to have optional parameters, and therefore the need to implement a new function for each possible combination of parameters, combined with the fact it can't overload like Java (each function has its own unique, hopefully-descriptive name), is that here we're just removing whole functions, rather than changing existing function signatures. Less of a breaking change for end-users.