[shims] Remove the _SwiftXCTestOverlayShims module
This used to be a private implementation detail of the XCTest overlay.
This module has been unused since the Xcode 12.5 release, but we temporarily kept it in the toolchain in case a downloadable toolchain gets used with an older Xcode. (The module used to be imported by the XCTest overlay interface, so its removal would have made it tricky to import XCTest in that scenario.)
Including the 13.3 beta, there have now been seven Xcode releases since the last one that imported this module, and the compiler/stdlib repository no longer supports building with Xcode 12.4 or below. It seems reasonable to stop installing this module on the toolchain now.
rdar://80552912
