macos: remove unused hosting window helper

Remove the unused SwiftUI environment key intended to expose a hosting
window. Nothing sets or reads the value.

macos/Sources/Helpers/HostingWindow.swift

@@ -1,14 +0,0 @@
-import SwiftUI
-
-struct HostingWindowKey: EnvironmentKey {
-    typealias Value = () -> NSWindow? // needed for weak link
-    static let defaultValue: Self.Value = { nil }
-}
-
-extension EnvironmentValues {
-    /// This can be used to set the hosting NSWindow to a NSHostingView
-    var hostingWindow: HostingWindowKey.Value {
-        get { return self[HostingWindowKey.self] }
-        set { self[HostingWindowKey.self] = newValue }
-    }
-}