Cleanup: Assets: Remove now unused AssetHandle type

`AssetHandle` was a hacky design solution that was never meant to be
permanent. `AssetRepresentation` is a better designed type superseding
it. A lot of work was put into relying less and less on `AssetHandle`,
now it's finally unused.

Final step for #108806.

source/blender/editors/asset/ED_asset_list.hh

@@ -10,7 +10,6 @@
 
 #include "BLI_function_ref.hh"
 
-struct AssetHandle;
 struct AssetLibraryReference;
 struct bContext;
 struct ID;
@@ -39,12 +38,6 @@ asset_system::AssetLibrary *library_get_once_available(
 
 /** Can return false to stop iterating. */
 using AssetListIterFn = FunctionRef<bool(asset_system::AssetRepresentation &)>;
-
-/**
- * \note This override avoids the file caching system, so it's more performant and avoids pitfalls
- * from the other override. Prefer this when access to #AssetRepresentation is enough, and no
- * #AssetHandle is needed.
- */
 void iterate(const AssetLibraryReference &library_reference, AssetListIterFn fn);
 
 /**

source/blender/makesdna/DNA_asset_types.h

@@ -197,23 +197,6 @@ typedef struct AssetWeakReference {
 #endif
 } AssetWeakReference;
 
-/**
- * To be replaced by #AssetRepresentation!
- *
- * Not part of the core design, we should try to get rid of it. Only needed to wrap FileDirEntry
- * into a type with PropertyGroup as base, so we can have an RNA collection of #AssetHandle's to
- * pass to the UI.
- *
- * \warning Never store this! When using #blender::ed::asset::list::iterate(), only access it
- * within the iterator function. The contained file data can be freed since the file cache has a
- * maximum number of items.
- */
-#
-#
-typedef struct AssetHandle {
-  const struct FileDirEntry *file_data;
-} AssetHandle;
-
 struct AssetCatalogPathLink {
   struct AssetCatalogPathLink *next, *prev;
   char *path;