Remove extra not null assertions (#202201)

* Remove extra not null assertions

With https://github.com/microsoft/TypeScript/pull/56908, TS should better preserve type refinements. To help test this out, I made a quick pass through our code to remove type assertions that are no longer needed

Most of these are not impacted by https://github.com/microsoft/TypeScript/pull/56908 but removing them helps TS test changes like this

Not adding an eslint rule for now as it requires whole program intellisense, which is too slow for commit hooks

* Fix merge

src/vs/workbench/api/browser/mainThreadDebugService.ts

@@ -92,7 +92,7 @@ export class MainThreadDebugService implements MainThreadDebugServiceShape, IDeb
 				const dto: IThreadFocusDto = {
 					kind: 'thread',
 					threadId: thread?.threadId,
-					sessionId: session!.getId(),
+					sessionId: session.getId(),
 				};
 				this._proxy.$acceptStackFrameFocus(dto);
 			}