Simplify constraints on operator co_await for SingleTypedSenders

Summary: Because of the guard against constraint recursion in the definition of `struct awaitable_sender_traits` (awaitable/concepts.h, line 75), there is no need to be so conservative with the constraints of the generic `operator co_await` for `SingleTypedSender`s.

Reviewed By: kirkshoop

Differential Revision: D14816672

fbshipit-source-id: b2f49588ae98ea63a24f4044692470c59be15e29

folly/experimental/pushmi/awaitable/senders.h

@@ -180,7 +180,7 @@ namespace awaitable_senders {
   // from within the body of the awaiting coroutine, do:
   // `using namespace ::folly::pushmi::awaitable_senders;`
   PUSHMI_TEMPLATE(class From)( //
-    requires not lazy::Awaiter<From> && lazy::SingleTypedSender<From> //
+    requires not Awaiter<From> && SingleTypedSender<From> //
   ) //
   sender_awaiter<From> operator co_await(From&& from) {
     return static_cast<From&&>(from);