fees: Complete the removal of fee-estimation file read code for old versions #15187

Complete the removal of fee-estimation file read code for old versions.

This completes the removal started by @TheBlueMatt in #11951:

0.15.0 introduced a new feeest file format, and support for parsing old versions was never fully added. We now simply fail to read the old format, so remove the dead partial-implementation.

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #13990 (Allow fee estimation to work with lower fees by ajtowns)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

I'm not convinced this change is necessary. It doesn't really remove any code, just the version argument, which might be useful in case a new format is introduced in the future.

@laanwj OK, closing!

That up to you. To be clear: my opinion here is not a strong NACK, and it's possible others think differently about it.

@laanwj I'll let others chime in and re-open if requested :-)