Removing all keys from `ThreadContextMap` does not clear the `ThreadLocal`

The implementations of [`ThreadContextMap#remove`](https://logging.apache.org/log4j/2.x/javadoc/log4j-api/org/apache/logging/log4j/spi/ThreadContextMap#remove(java.lang.String)) and [`CleanableThreadContextMap#removeAll`](https://logging.apache.org/log4j/2.x/javadoc/log4j-api/org/apache/logging/log4j/spi/CleanableThreadContextMap#removeAll(java.lang.Iterable)) in the `DefaultThreadContextMap` and `CopyOnWriteSortedArrayThreadContextMap` implementations do not clear the underlying `ThreadLocal`, after all keys have been removed from the context map.

This issue prevents `CopyOnWriteSortedArrayThreadContextMap` to be safely used in a web application, if code like this is used:

```java
ThreadContext.put("key", "value");
try {
...
} finally {
ThreadContext.remove("key");
}
```

**Remark**: this issue does not affect the `GarbageFreeSortedArrayThreadContextMap` implementation, since leaving a non-null value of `ThreadContext` is part of its garbage-free design.