Remove cross-contaminated #827 test from PropertyMemberTests.cs

The TestOverridableAutoPropertyBackingFieldAccessAsync test (issue #827)
was mistakenly added to this branch, which only contains the fix for
issue #557. The #827 fix code is in a separate PR (#1252) and is not
present here, so the test would always fail.

https://claude.ai/code/session_01AkwUvu3XuCdj3D4axoX4UX

Author: claude

Tests/CSharp/MemberTests/PropertyMemberTests.cs

@@ -875,40 +875,4 @@ public static IEnumerable<object[]> SomeObjects
 }");
     }
 
-    /// <summary>Issue #827: VB auto-property backing field access via _Prop should map to MyClassProp for overridable properties</summary>
-    [Fact]
-    public async Task TestOverridableAutoPropertyBackingFieldAccessAsync()
-    {
-        await TestConversionVisualBasicToCSharpAsync(
-            @"Class Foo
-    Overridable Property Prop As Integer = 5
-    Sub Test()
-        _Prop = 10
-        Dim x = _Prop
-    End Sub
-End Class", @"
-internal partial class Foo
-{
-    public int MyClassProp { get; set; } = 5;
-
-    public virtual int Prop
-    {
-        get
-        {
-            return MyClassProp;
-        }
-
-        set
-        {
-            MyClassProp = value;
-        }
-    }
-
-    public void Test()
-    {
-        MyClassProp = 10;
-        int x = MyClassProp;
-    }
-}");
-    }
 }