remove lvalue-qual Future::reduce()

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

Reviewed By: yfeldblum

Differential Revision: D9329242

fbshipit-source-id: 4f192863e69c12484578db6f756de82b97c4427b

folly/futures/Future.h

@@ -1790,12 +1790,6 @@ class Future : private futures::detail::FutureBase<T> {
   template <class I, class F>
   Future<I> reduce(I&& initial, F&& func) &&;
 
-  template <class I, class F>
-  Future<I> reduce(I&& initial, F&& func) & {
-    return std::move(*this).reduce(
-        std::forward<I>(initial), std::forward<F>(func));
-  }
-
   /// Create a Future chain from a sequence of continuations. i.e.
   ///
   /// ```