Fix joining threads twice when exiting EventBaseTest.RunInThread normally

Summary:
10e9cd3e set up SCOPE_EXIT to join threads (wait for them to exit) on
exceptions. However, it is also called on normal exit. Remove the
original code at the end for joining threads.

This fixes EventBaseTest regression.
Closes https://github.com/facebook/folly/pull/468

Reviewed By: yfeldblum

Differential Revision: D3792977

Pulled By: Orvid

fbshipit-source-id: e4b12202ae3e35fd942ad2ca1d8d405cf1a28f68

folly/io/async/test/EventBaseTest.cpp

@@ -1176,11 +1176,6 @@ TEST(EventBaseTest, RunInThread) {
   for (uint32_t n = 0; n < numThreads; ++n) {
     ASSERT_EQ(expectedValues[n], opsPerThread);
   }
-
-  // Wait on all of the threads.
-  for (auto& thread: threads) {
-    thread.join();
-  }
 }
 
 //  This test simulates some calls, and verifies that the waiting happens by