Remove dead includes in folly/lang

Reviewed By: yfeldblum

Differential Revision: D18614792

fbshipit-source-id: 9b77ae8aba9153c9051dd5935cda807dfad4e6e2

folly/lang/test/BitsBenchmark.cpp

@@ -26,7 +26,6 @@
 
 #include <folly/Benchmark.h>
 
-using namespace folly;
 
 BENCHMARK(nextPowTwoClz, iters) {
   for (unsigned long i = 0; i < iters; ++i) {

folly/lang/test/LaunderTest.cpp

@@ -17,7 +17,6 @@
 #include <folly/lang/Launder.h>
 #include <folly/portability/GTest.h>
 
-using namespace ::testing;
 using namespace folly;
 
 TEST(LaunderTest, Basics) {

folly/lang/test/SafeAssertTest.cpp

@@ -21,7 +21,6 @@
 #include <folly/Benchmark.h>
 #include <folly/portability/GTest.h>
 
-using namespace folly;
 
 // clang-format off
 [[noreturn]] void fail() {