Revert #124 and #135 (#139)

This seems to be causing a lot of weirdness, presumably because there's
another missing thing like in #135, but I don't know what it might be and
it feels like it might take a while to find. Backing these changes out
to fix develop while we sort it out.

Fixes https://github.com/element-hq/element-web/issues/28179

test/stores/room-list/MessagePreviewStore-test.ts

@@ -80,7 +80,7 @@ describe("MessagePreviewStore", () => {
         mocked(client.getRoom).mockReturnValue(room);
 
         store = MessagePreviewStore.testInstance();
-        await store.useUnitTestClient(client);
+        await store.start();
         await setupAsyncStoreWithClient(store, client);
     });