fix: minor polishing - found when implementing it in SPV-Wallet

[wregulski]

This pull request includes several changes to improve code readability and functionality in the primitives/ec and script packages. The most important changes include removing unused variables, fixing a loop initialization, and renaming a function for clarity.

Code readability and cleanup:

• primitives/ec/field.go: Removed the unused fieldOverflowBits variable to clean up the code.

Code clarity:

• script/script_chunk.go: Renamed the DecodeStringParts function to DecodeScriptHex for better clarity and to accurately reflect its purpose.

Linked Issues / Tickets

No references to specific tasks (maintenance/feedback).

• [] I have added new unit tests
• All tests pass locally
• I have tested manually in my local environment

Checklist:

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have updated CHANGELOG.md with my changes
• I have run npm run doc and npm run lint one final time before requesting a review