[NFC] CoroutineAccessors: Remove old runtime files

The functions are open-coded now.

Author: nate-chandler

include/swift/Runtime/Coro.h

@@ -20,7 +20,6 @@
 #include "TaskPrivate.h"
 #include "swift/ABI/Task.h"
 #include "swift/Runtime/Concurrency.h"
-#include "swift/Runtime/Coro.h"
 
 #include <stdlib.h>
 

stdlib/public/Concurrency/TaskAlloc.cpp

@@ -38,7 +38,6 @@ set(swift_runtime_sources
     Bincompat.cpp
     BytecodeLayouts.cpp
     Casting.cpp
-    Coro.cpp
     CrashReporter.cpp
     Demangle.cpp
     DynamicCast.cpp