refactor(acir): remove unused done flag

[Snezhkko]

The ACIR text lexer stored a done flag that was only written at EOF and never read, unlike the frontend lexer where this flag drives the Iterator implementation. The ACIR parser relies solely on the Eof token to detect end of input, so keeping the flag adds unnecessary state and can confuse future maintainers. This change removes the dead done field, its initialization, and the EOF assignment without altering lexer or parser behavior.

[github-actions]

Thank you for your contribution to the Noir language.

Please do not force push to this branch after the Noir team have started review of this PR. Doing so will only delay us merging your PR as we will need to start the review process from scratch.

Thanks for your understanding.