Remove unused includes from AsyncLogWriter.cpp

Summary:
Removed three unused include directives from `fbcode/folly/logging/AsyncLogWriter.cpp`:
- `folly/Exception.h`
- `folly/FileUtil.h`
- `folly/Portability.h`

None of these headers provide symbols used in this translation unit.

Reviewed By: r-barnes

Differential Revision: D95404233

fbshipit-source-id: 72278f9e52f09304c01284a9b79effcfaa00fe60

Author: yfeldblum

third-party/folly/src/folly/logging/AsyncLogWriter.cpp

@@ -16,9 +16,6 @@
 
 #include <folly/logging/AsyncLogWriter.h>
 
-#include <folly/Exception.h>
-#include <folly/FileUtil.h>
-#include <folly/Portability.h>
 #include <folly/logging/LoggerDB.h>
 #include <folly/system/AtFork.h>
 #include <folly/system/ThreadName.h>