get rid of redundant calls to RequestContext::saveContext()

get rid of redundant calls to RequestContext::saveContext()

Summary:
In the past, these calls were required.

Used to solve static destruction ordering issue.  Any static object that uses RequestContext must call this function in its constructor.

That is when we were using `static folly::ThreadLocal<std::shared_ptr<RequestContext>>`, which was non-leaky.

The problem being addressed is when we have some code of the form:

```lang=c++
void doWork() {
  static EventBase eb;
}
```

But now we are using `SingletonThreadLocal<std::shared_ptr<RequestContext>>`, which is leaky.

So the issue that these calls were there to address seems to have been resolved.

Reviewed By: yfeldblum

Differential Revision: D6332597

fbshipit-source-id: c6aba6620ef2fb3a344ea20f56c8b9c0cdf42c70