Simpler PickTaskWrapper fallback logic

Summary:
ispeters correctly points out that the "old compiler fallback" doesn't need the `<unsafe, false>` specialization, since that's redundant with its more generic `<Safety, false>` specialization, which has identical consequences.

There's no real problem with the old code IIUC, but this is slightly easier to read.

Reviewed By: ispeters

Differential Revision: D77848551

fbshipit-source-id: c98a854ef832970b57c62b8ef080525c0e6770ad

Author: snarkmaster

third-party/folly/src/folly/coro/detail/PickTaskWrapper.h

@@ -62,6 +62,8 @@ struct identity_metafunction {
 template <safe_alias, bool /*must await immediately (now)*/>
 struct PickTaskWrapperImpl;
 
+#if FOLLY_HAS_IMMOVABLE_COROUTINES
+
 template <>
 struct PickTaskWrapperImpl<safe_alias::unsafe, /*await now*/ false> {
   template <typename T>
@@ -70,8 +72,6 @@ struct PickTaskWrapperImpl<safe_alias::unsafe, /*await now*/ false> {
   using TaskWithExecutor = TaskWithExecutor<T>;
 };
 
-#if FOLLY_HAS_IMMOVABLE_COROUTINES
-
 template <>
 struct PickTaskWrapperImpl<safe_alias::unsafe, /*await now*/ true> {
   template <typename T>