Remove unnecessary return values from Storage{Read, Write} methods

[Dentosal]

When reviewing this fix FuelLabs/fuel-core#2673, it became apparent that the return values from StorageRead and StorageWrite traits are not useful and quite misleading. We require the target buffer to be filled completely, which means the caller already knows the number of bytes written after successful invocation.

The PR removes the return values indicating number of bytes written. It's a breaking change, but the fix on caller side is simple and likely affects fuel-core and execution-tracing only.

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