[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 https://github.com/apple/swift-corelibs-libdispatch/pull/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.
