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

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).thenMulti(...)` instead of `f.thenMulti(...)`. This syntactic change in the callsites forces callers to acknowledge the method's rvalue semantics.

Reviewed By: LeeHowes

Differential Revision: D9437627

fbshipit-source-id: f62dc095ce57904057a97c83c958de5227a112a8

folly/futures/Future.h

@@ -1797,12 +1797,6 @@ class Future : private futures::detail::FutureBase<T> {
         .thenMulti(std::forward<Callbacks>(fns)...);
   }
 
-  template <class Callback, class... Callbacks>
-  auto thenMulti(Callback&& fn, Callbacks&&... fns) & {
-    return std::move(*this).thenMulti(
-        std::forward<Callback>(fn), std::forward<Callbacks>(fns)...);
-  }
-
   /// Create a Future chain from a sequence of callbacks.
   ///
   /// Preconditions: