Delete backdeploy concurrency library

This library served its purpose and has overstayed its welcome.
The library shipped in Xcode can't change again from the state it was
in, so changes to the sources here will not affect that library, which
is confusing. The library does not currently build due to changes in the
runtime headers, so it does not give meaningful signal to anything
anymore. If you need to test things in a backdeploy concurrency
environment, use the copy from the toolchain in Xcode as that will give
you a far clearer picture of what the code will actually be running with
than the state of the sources here did.

Author: etcwilde

CMakeLists.txt

@@ -1289,11 +1289,6 @@ else()
     endif()
   endif()
 
-  if (BUILD_SWIFT_CONCURRENCY_BACK_DEPLOYMENT_LIBRARIES)
-    # Build the back-deployed concurrency library.
-    add_subdirectory(stdlib/public/BackDeployConcurrency)
-  endif()
-
   # Some tools (e.g. swift-reflection-dump) rely on a host swiftRemoteInspection,
   # so ensure we build that when building tools.
   if(SWIFT_INCLUDE_TOOLS OR SWIFT_BUILD_STDLIB_CXX_MODULE)