Don't explicitly alias dynamic after using the entire folly namespace

Summary: Because, to the best of my knowledge, MSVC is quite right to claim it doesn't know which `dynamic` everyone keeps referring to. Is it `folly::dynamic` or `::dynamic`?

Reviewed By: yfeldblum

Differential Revision: D4209535

fbshipit-source-id: 63322a8f960954e1a63fe5c9a4a5d9395c7827d9

folly/experimental/test/DynamicParserTest.cpp

@@ -28,7 +28,6 @@
 #include <folly/portability/GTest.h>
 
 using namespace folly;
-using dynamic = folly::dynamic;
 
 // NB Auto-conversions are exercised by all the tests, there's not a great
 // reason to test all of them explicitly, since any uncaught bugs will fail