fix(SecretKey): remove Default implementation

[rymnc]

Fixes #830

Description

Deriving Default for the SecretKey doesn't exactly make too much sense given that we don't use it anywhere in this codebase, and the fact that k256::SecretKey throws an error if the provided byte slice is 0 -

We could have a default impl by using rng, but we should expect consumers of this library to use their own impl, or use SecretKey::random()

Checklist

• Breaking changes are clearly marked as such in the PR description and changelog
• New behavior is reflected in tests
• If performance characteristic of an instruction change, update gas costs as well or make a follow-up PR for that
• The specification matches the implemented behavior (link update PR if changes are needed)

Before requesting review

• I have reviewed the code myself
• I have created follow-up issues caused by this PR and linked them here

After merging, notify other teams

[Add or remove entries as needed]

• Rust SDK
• Sway compiler
• Platform documentation (for out-of-organization contributors, the person merging the PR will do this)
• Someone else?

[rafal-ch]

LGTM 👍

[xgreenx]

It would be nice to add a default implementation under the test-helpers and rand features. In this case in tests we can generate them easily

[rymnc]

It would be nice to add a default implementation under the test-helpers and rand features. In this case in tests we can generate them easily

addressed in 8a34cd6