fix: remove required legacy field

[aeneasr]

Closes #2408

Related issue(s)

Checklist

• I have read the contributing guidelines.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I am following the
  contributing code guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security. vulnerability, I
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  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further Comments

[codecov]

Codecov Report

Merging #2410 (a40ed32) into master (a42a0f7) will not change coverage.
The diff coverage is 0.00%.

❗ Current head a40ed32 differs from pull request most recent head 270cb73. Consider uploading reports for the commit 270cb73 to get more accurate results

@@           Coverage Diff           @@
##           master    #2410   +/-   ##
=======================================
  Coverage   76.62%   76.62%           
=======================================
  Files         315      315           
  Lines       17358    17358           
=======================================
  Hits        13300    13300           
  Misses       3122     3122           
  Partials      936      936           

Impacted Files	Coverage Δ
selfservice/strategy/oidc/provider_config.go	37.20% <0.00%> (ø)

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