Remove the ShouldNotDeallocateInCallee bit from AsyncContextFlags.

The original async ABI made callees deallocate the context,
which allows tail calls (at the async-function level) but
interferes with callers' ability to optimize callee frame
allocation.  The purpose of this bit was to allow callers
to do that optimization, but we've since just made callers
responsible for deallocating the context, which is overall
just a lot simpler.  So this has been dead for quite some
time.

Author: rjmccall

include/swift/ABI/MetadataValues.h

@@ -2211,7 +2211,6 @@ class AsyncContextFlags : public FlagSet<uint32_t> {
     Kind_width          = 8,
 
     CanThrow            = 8,
-    ShouldNotDeallocate = 9,
 
     // Kind-specific flags should grow down from 31.
 
@@ -2231,18 +2230,6 @@ class AsyncContextFlags : public FlagSet<uint32_t> {
   /// Whether this context is permitted to throw.
   FLAGSET_DEFINE_FLAG_ACCESSORS(CanThrow, canThrow, setCanThrow)
 
-  /// Whether a function should avoid deallocating its context before
-  /// returning.  It should still pass its caller's context to its
-  /// return continuation.
-  ///
-  /// This flag can be set in the caller to optimize context allocation,
-  /// e.g. if the callee's context size is known statically and simply
-  /// allocated as part of the caller's context, or if the callee will
-  /// be called multiple times.
-  FLAGSET_DEFINE_FLAG_ACCESSORS(ShouldNotDeallocate,
-                                shouldNotDeallocateInCallee,
-                                setShouldNotDeallocateInCallee)
-
   /// See AsyncContinuationFlags::isExecutorSwitchForced.
   FLAGSET_DEFINE_FLAG_ACCESSORS(Continuation_IsExecutorSwitchForced,
                                 continuation_isExecutorSwitchForced,

stdlib/public/Concurrency/Task.cpp

@@ -629,7 +629,6 @@ static AsyncTaskAndContext swift_task_create_commonImpl(
   // be is the final hop.  Store a signed null instead.
   initialContext->Parent = nullptr;
   initialContext->Flags = AsyncContextKind::Ordinary;
-  initialContext->Flags.setShouldNotDeallocateInCallee(true);
 
   // Attach to the group, if needed.
   if (group) {