Rm std::unique_ptr null-deleter test case

Summary:
The null-deleter test case showcases not_null's handling of null deleters. It also tries to demonstrate that std::unique_ptr does not handle null deleters properly (e.g. by segfaulting). The latter is really "check what kind of undefined behavior we encounter", and this doesn't always work. Surprise! Specifically, the undefined behavior encountered when supplying a null deleter into std::unique_ptr when using tsan is an infinite hang, which breaks the test suite.

Fortunately, testing std::unique_ptr's handling of null deleters is irrelevant to the correctness of not_null_unique_ptr, so we can remove this code.

After this diff, the following test does not hang:
  buck test mode/dev-tsan //folly/memory/test:not_null_test -- 'nn\.null_deleter' --run-disabled

Reviewed By: yfeldblum

Differential Revision: D24662902

fbshipit-source-id: 230187890b29e1d1a5c8284958c9b798c891c7f7

folly/memory/test/not_null_test.cpp

@@ -500,12 +500,6 @@ TEST(nn, hash) {
 }
 
 TEST(nn, null_deleter) {
-  int* i = new int(5);
-  auto f = [&]() { std::unique_ptr<int, void (*)(int*)> nnui(i, nullptr); };
-  // Error messages are different in build modes, but both die
-  EXPECT_DEATH(f(), "");
-  delete i;
-
   int* j = new int(6);
   try {
     not_null_unique_ptr<int, void (*)(int*)> nnui(j, nullptr);