api: remove ::dummy from Id types

`::dummy` was used for testing purposes and became redundant after
adding e.g. `from_raw` and `into_raw` methods on `Id` types.

src/platform_impl/web/window.rs

@@ -433,10 +433,6 @@ impl Drop for Inner {
 pub struct WindowId(pub(crate) u64);
 
 impl WindowId {
-    pub const fn dummy() -> Self {
-        Self(0)
-    }
-
     pub const fn into_raw(self) -> u64 {
         self.0
     }