remove unnecessary & deprecated activate() call

Summary: let's not do this to ourselves :)

Test Plan: unit

Reviewed By: yfeldblum@fb.com

Subscribers: folly-diffs@, jsedgwick, yfeldblum, chalfant

FB internal diff: D1993491

Signature: t1:1993491:1429063112:b8664b365ad12fa4b51e9bd7fde9e4d197dbc648

folly/futures/test/ExecutorTest.cpp

@@ -179,7 +179,7 @@ class CrappyExecutor : public Executor {
 TEST(Executor, CrappyExecutor) {
   CrappyExecutor x;
   try {
-    auto f = Future<void>().via(&x).activate().then([](){
+    auto f = Future<void>().via(&x).then([](){
       return;
     });
     f.value();