Update LINQ tutorial sample code to match tutorial progression

[Copilot]

The sample code for the Working with LINQ tutorial was using an enum-based approach that didn't match what the tutorial teaches. This created confusion for learners who expected to see the string-based code from the tutorial when they referenced the "completed sample."

Problem:

• Tutorial teaches string-based Suits() and Ranks() methods using yield return statements
• Sample code used enum-based Suit and Rank types with Enum.GetValues()
• Output format was different: tutorial shows { Suit = clubs, Rank = two } while sample showed { Suit = Clubs, Rank = Two }

Changes:

• Replaced enum declarations with string-based iterator methods:

  static IEnumerable<string> Suits()
  {
      yield return "clubs";
      yield return "diamonds";
      yield return "hearts";
      yield return "spades";
  }

• Updated method signatures to return IEnumerable<string> instead of enum types
• Simplified Main method to directly use the string-based methods without null checks
• Removed unused playingcard.cs file that wasn't part of the string-based tutorial approach

The sample code now perfectly matches the tutorial's step-by-step progression, eliminating confusion when learners check the completed sample for reference.

Fixes #28865.

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[BillWagner]

This isn't even close. Closing.