remove lvalue-qual Future::unit()

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

Reviewed By: yfeldblum

Differential Revision: D9329200

fbshipit-source-id: e8a60498f5ff8a15700c7b0fb0aba5a6faf83da1

folly/futures/Future.h

@@ -1400,9 +1400,6 @@ class Future : private futures::detail::FutureBase<T> {
   Future<Unit> unit() && {
     return std::move(*this).then();
   }
-  Future<Unit> unit() & {
-    return std::move(*this).then();
-  }
 
   /// Set an error continuation for this Future. The continuation should take an
   /// argument of the type that you want to catch, and should return a value of