[Concurrency] Remove the C++ runtime test for swift_future_task_wait.

Setting up the appropriate async calling convention is a mess. We're
better off reimplementing this test purely in Swift.

Author: DougGregor

unittests/runtime/CMakeLists.txt

@@ -55,7 +55,6 @@ if(("${SWIFT_HOST_VARIANT_SDK}" STREQUAL "${SWIFT_PRIMARY_VARIANT_SDK}") AND
 
   if(SWIFT_ENABLE_EXPERIMENTAL_CONCURRENCY)
     list(APPEND PLATFORM_SOURCES
-      TaskFuture.cpp
       TaskStatus.cpp
       )
     list(APPEND PLATFORM_TARGET_LINK_LIBRARIES
@@ -67,7 +66,6 @@ if(("${SWIFT_HOST_VARIANT_SDK}" STREQUAL "${SWIFT_PRIMARY_VARIANT_SDK}") AND
   set(LLVM_OPTIONAL_SOURCES
     weak.mm
     Refcounting.mm
-    TaskFuture.cpp
     TaskStatus.cpp)
 
   add_swift_unittest(SwiftRuntimeTests