Remove dead includes in folly/stats

Reviewed By: yfeldblum

Differential Revision: D19328769

fbshipit-source-id: 5d929eaa3bf4e2d0a63b8f1d95a613fee68525b4

folly/stats/detail/test/RadixSortTest.cpp

@@ -19,7 +19,6 @@
 #include <folly/Random.h>
 #include <folly/stats/detail/DoubleRadixSort.h>
 
-using namespace folly;
 using namespace folly::detail;
 
 TEST(DoubleRadixSort, Basic) {

folly/stats/test/SlidingWindowTest.cpp

@@ -18,7 +18,6 @@
 
 #include <folly/portability/GTest.h>
 
-using namespace folly;
 using namespace folly::detail;
 
 class SlidingWindowTest : public ::testing::Test {

folly/stats/test/TimeseriesHistogramTest.cpp

@@ -20,7 +20,6 @@
 
 #include <folly/portability/GTest.h>
 
-using namespace std;
 using namespace folly;
 using std::chrono::seconds;