[TaskLocal] Remove comment which is true, but is confusing to end users

because it discusses an implementation detail of the type the comment is
on. Instead, when developers read comments they think about what they
can store inside a task local -- so the comment about "task local must
be a class" led them to believe THEIR types must be classes, while the
comment was explaining the implementation detail of `class
TaskLocal<T>`.

Resolves rdar://117982608

Author: ktoso

stdlib/public/Concurrency/TaskLocal.swift

@@ -90,9 +90,6 @@ import Swift
 ///     func call() {
 ///       print("traceID: \(traceID)") // 1234
 ///     }
-///
-/// This type must be a `class` so it has a stable identity, that is used as key
-/// value for lookups in the task local storage.
 @propertyWrapper
 @available(SwiftStdlib 5.1, *)
 public final class TaskLocal<Value: Sendable>: Sendable, CustomStringConvertible {