remove lvalue-qual Future::onTimeout(...)

Summary:
This is part of "the great r-valuification of folly::Future":
* This is something we should do for safety in general.
* Several of folly::Future's methods are lvalue-qualified even though they act as though they are rvalue-qualified, that is, they provide a postcondition that says, in effect, callers should act as though the method invalidated its `this` object (regardless of whether that invalidation was actual or logical).
* This violates the C++ principle to "Express ideas directly in code" (see Core Guidelines), and generally makes it more confusing for callers as well as hiding the actual semantics from tools (linters, compilers, etc.).
* This dichotomy and confusion has manifested itself by some failures around D7840699 since lvalue-qualification hides that operation's move-out semantics - leads to some use of future operations that are really not correct, but are not obviously incorrect.
* The goal of rvalueification is to make sure methods that are logically rvalue-qualified are actually rvalue-qualified, which forces callsites to acknowledge that rvalueification, e.g., `std::move(f).onTimeout(...)` instead of `f.onTimeout(...)`. This syntactic change in the callsites forces callers to acknowledge the method's rvalue semantics.

Reviewed By: LeeHowes

Differential Revision: D9441979

fbshipit-source-id: a2f3416bdf481d32d1b94cde1bb713e1afe8ab67

folly/futures/Future.h

@@ -1519,11 +1519,6 @@ class Future : private futures::detail::FutureBase<T> {
   template <class F>
   Future<T> onTimeout(Duration, F&& func, Timekeeper* = nullptr) &&;
 
-  template <class F>
-  Future<T> onTimeout(Duration dur, F&& func, Timekeeper* tk = nullptr) & {
-    return std::move(*this).onTimeout(dur, std::forward<F>(func), tk);
-  }
-
   /// Throw FutureTimeout if this Future does not complete within the given
   /// duration from now. The optional Timekeeper is as with futures::sleep().
   ///