Debt: Remove legacy required ValidationIssue filter

[bjosttveit]

Description

Assuming that v4 will require an app-lib version >= 7.14.0 we can safely remove this legacy filter for backend required-validations.

Related Issue(s)

• Add Source to ValidationIssue app-lib-dotnet#287
• Filter validations from backend based on source field #1386

Verification/QA

• Manual functionality testing
  • I have tested these changes manually
  • Creator of the original issue (or service owner) has been contacted for manual testing (or will be contacted when released in alpha)
  • No testing done/necessary
• Automated tests
  • Unit test(s) have been added/updated
  • Cypress E2E test(s) have been added/updated
  • No automatic tests are needed here (no functional changes/additions)
  • I want someone to help me make some tests
• UU/WCAG (follow these guidelines until we have our own)
  • I have tested with a screen reader/keyboard navigation/automated wcag validator
  • No testing done/necessary (no DOM/visual changes)
  • I want someone to help me perform accessibility testing
• User documentation @ altinn-studio-docs
  • Has been added/updated
  • No functionality has been changed/added, so no documentation is needed
  • I will do that later/have created an issue
• Support in Altinn Studio
  • Issue(s) created for support in Studio
  • This change/feature does not require any changes to Altinn Studio
• Sprint board
  • The original issue (or this PR itself) has been added to the Team Apps project and to the current sprint board
  • I don't have permissions to do that, please help me out
• Labels
  • I have added a kind/* label to this PR for proper release notes grouping
  • I don't have permissions to add labels, please help me out

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

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[olemartinorg]

That might be an assumption, but I made a note of it in #339. I don't know of any way for us to check the backend version and let the developers know - so we either have to just document it (and hope everyone reads that) or live with some people mistakenly reporting this as a bug.