Remove test struct with nested field ref

- Can't use nested field refs with .NET 4.8.

***NO_CI***

Author: josesimoes

Tests/NFUnitTestSystemLib/RuntimeHelpersTests.cs

@@ -23,7 +23,7 @@ public static void IsReferenceOrContainsReferences()
             Assert.IsTrue(RuntimeHelpers.IsReferenceOrContainsReferences<Span<char>>());
             Assert.IsTrue(RuntimeHelpers.IsReferenceOrContainsReferences<ReadOnlySpan<char>>());
             //Assert.IsTrue(RuntimeHelpers.IsReferenceOrContainsReferences<RefStructWithRef>());
-            Assert.IsTrue(RuntimeHelpers.IsReferenceOrContainsReferences<RefStructWithNestedRef>());
+            //Assert.IsTrue(RuntimeHelpers.IsReferenceOrContainsReferences<RefStructWithNestedRef>());
         }
 
         private struct StructWithoutReferences
@@ -60,9 +60,9 @@ private ref struct RefStructWithReferences
         //    }
         //}
 
-        private ref struct RefStructWithNestedRef
-        {
-            public Span<char> a;
-        }
+        //private ref struct RefStructWithNestedRef
+        //{
+        //    public Span<char> a;
+        //}
     }
 }