[shims] Remove the _SwiftDispatchOverlayShims module

This module used to be a private implementation detail of the Distpatch overlay on Apple platforms, and swift-corelibs-libdispatch elsewhere.

Darwin hasn't been using this module for a couple years now, and swift-corelibs-dispatch stopped using it with swiftlang/swift-corelibs-libdispatch#592. We are now free to remove it from the Swift toolchain -- which is good, as it contains declarations that are specific to Dispatch, and the compiler/stdlib is not the right place to maintain them.

Author: lorentey

stdlib/public/SwiftShims/CMakeLists.txt

@@ -24,8 +24,6 @@ set(sources
   _SwiftConcurrency.h
   _SwiftDistributed.h
 
-  DispatchOverlayShims.h
-
   module.modulemap
   )
 set(output_dir "${SWIFTLIB_DIR}/shims")

stdlib/public/SwiftShims/DispatchOverlayShims.h

@@ -30,15 +30,3 @@ module SwiftOverlayShims {
   header "LibcOverlayShims.h"
   export *
 }
-
-
-// Obsolete overlay shims on Apple platforms.
-// Note: These deliberately do not use "export *" to avoid circularity issues
-// (overlay -> shims -> framework -> overlay)
-
-// NOTE: These are obsolete and should never be imported into any project.
-// They are going to be removed in a future compiler release.
-
-module _SwiftDispatchOverlayShims {
-  header "DispatchOverlayShims.h"
-}