remove extern "C" (#1416)

Summary:
The upstream [liburing](https://git.kernel.dk/cgit/liburing/tree/src/include/liburing.h) has changed to use ```extern "C" ```, we must remove the duplicated `extern "C"`.

Pull Request resolved: https://github.com/facebook/folly/pull/1416

Differential Revision: D22752570

Pulled By: yfeldblum

fbshipit-source-id: fa62d8e462e04676af6d563225455400d7e83c4b

folly/experimental/io/IoUring.h

@@ -16,9 +16,7 @@
 
 #pragma once
 
-extern "C" {
 #include <liburing.h>
-}
 
 #include <folly/SharedMutex.h>
 #include <folly/experimental/io/AsyncBase.h>

folly/experimental/io/IoUringBackend.h

@@ -16,9 +16,7 @@
 
 #pragma once
 
-extern "C" {
 #include <liburing.h>
-}
 
 #include <folly/Function.h>
 #include <folly/Range.h>