notebooks: remove impure renderer API

This removes the initial notebook renderer API and keeps the 'pure'
renderer API described in #102644 and hacked-in previously.

Remaining work in this area, in no particular order:

- Add messaging context to postMessage as requested by Don (API proposal TBA)
- Cleanups around how state is managed internally in the backLayerWebView
- Deprecate the renderer `viewType` in favor of calling it the `id` or `rendererId`

Q: I kept around some of the "transform" functions since the mime type
picking happens there, not sure if there's a better place for this
to happen now, or whether these methods should simply be renamed.

src/vs/workbench/api/common/extHost.api.impl.ts

@@ -948,10 +948,6 @@ export function createApiFactoryAndRegisterActors(accessor: ServicesAccessor): I
 				checkProposedApiEnabled(extension);
 				return extHostNotebook.registerNotebookKernelProvider(extension, selector, provider);
 			},
-			registerNotebookOutputRenderer: (type: string, outputFilter: vscode.NotebookOutputSelector, renderer: vscode.NotebookOutputRenderer) => {
-				checkProposedApiEnabled(extension);
-				return extHostNotebook.registerNotebookOutputRenderer(type, extension, outputFilter, renderer);
-			},
 			get activeNotebookEditor(): vscode.NotebookEditor | undefined {
 				checkProposedApiEnabled(extension);
 				return extHostNotebook.activeNotebookEditor;