Remove FormatFStringPart

[dhruvmanila]

Summary

This is just a small refactor to remove the FormatFStringPart as it's only used in the case when the f-string is not implicitly concatenated in which case the only part is going to be FString. In implicitly concatenated f-strings, we use StringLike instead.

[github-actions]

ruff-ecosystem results

Formatter (stable)

✅ ecosystem check detected no format changes.

Formatter (preview)

✅ ecosystem check detected no format changes.