Remove customer tax id required for Poland

[mrdanwa]

After reviewing the KSeF FA(3) documentation, third-country buyers and consumers don't need a tax ID on the invoice.

Check invopop/gobl.ksef#56

Pre-Review Checklist

• Opened this PR as a draft
• Read the CONTRIBUTING.md guide.
• Performed a self-review of my code.
• Added thorough tests with at least 90% code coverage.
• Modified or created example GOBL documents to show my changes in use, if appropriate.
• Added links to the source of the changes in tax regimes or addons, either structured or in the comments.
• Run go generate . to ensure that the Schemas and Regime data are up to date.
• Reviewed and fixed all linter warnings.
• Been obsessive with pointer nil checks to avoid panics.
• Updated the CHANGELOG.md with an overview of my changes.
• Marked this PR as ready for review.

And if you are part of the org:

• Requested a review from Copilot and fixed or dismissed (with a reason) all the feedback raised.
• Requested a review from @samlown.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.09%. Comparing base (72beee4) to head (997f1d1).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #808   +/-   ##
=======================================
  Coverage   93.09%   93.09%           
=======================================
  Files         368      368           
  Lines       19624    19631    +7     
=======================================
+ Hits        18269    18276    +7     
  Misses        919      919           
  Partials      436      436           

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