revert D1985475, clang still borked

Summary:
This reverts commit bb08e2405dc68e6dab1f0066b15faa2821ad3dc7.

Test Plan: revert-hammer

Reviewed By: hans@fb.com

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

FB internal diff: D2006283

Signature: t1:2006283:1429556243:41e6b3189ce2da5a0f1a32a663ff8761628ca574

folly/futures/Future-inl.h

@@ -235,16 +235,6 @@ Future<T>::then(R(Caller::*func)(Args...), Caller *instance) {
   });
 }
 
-template <class T>
-template <class... Args>
-auto Future<T>::then(Executor* x, Args&&... args)
-  -> decltype(this->then(std::forward<Args>(args)...))
-{
-  auto oldX = getExecutor();
-  setExecutor(x);
-  return this->then(std::forward<Args>(args)...).via(oldX);
-}
-
 template <class T>
 Future<void> Future<T>::then() {
   return then([] (Try<T>&& t) {});

folly/futures/Future.h

@@ -325,22 +325,6 @@ class Future {
   Future<typename isFuture<R>::Inner>
   then(R(Caller::*func)(Args...), Caller *instance);
 
-  /// Execute the callback via the given Executor. The executor doesn't stick.
-  ///
-  /// Contrast
-  ///
-  ///   f.via(x).then(b).then(c)
-  ///
-  /// with
-  ///
-  ///   f.then(x, b).then(c)
-  ///
-  /// In the former both b and c execute via x. In the latter, only b executes
-  /// via x, and c executes via the same executor (if any) that f had.
-  template <class... Args>
-  auto then(Executor* x, Args&&... args)
-    -> decltype(this->then(std::forward<Args>(args)...));
-
   /// Convenience method for ignoring the value and creating a Future<void>.
   /// Exceptions still propagate.
   Future<void> then();

folly/futures/test/ViaTest.cpp

@@ -184,33 +184,3 @@ TEST(Via, chain3) {
   EXPECT_EQ(42, f.get());
   EXPECT_EQ(3, count);
 }
-
-TEST(Via, then2) {
-  ManualExecutor x1, x2;
-  bool a,b,c;
-  via(&x1)
-    .then([&]{ a = true; })
-    .then(&x2, [&]{ b = true; })
-    .then([&]{ c = true; });
-
-  EXPECT_FALSE(a);
-  EXPECT_FALSE(b);
-
-  x1.run();
-  EXPECT_TRUE(a);
-  EXPECT_FALSE(b);
-  EXPECT_FALSE(c);
-
-  x2.run();
-  EXPECT_TRUE(b);
-  EXPECT_FALSE(c);
-
-  x1.run();
-  EXPECT_TRUE(c);
-}
-
-TEST(Via, then2Variadic) {
-  struct Foo { void foo(Try<void>) {} };
-  Foo f;
-  makeFuture().then(nullptr, &Foo::foo, &f);
-}