Simplify local transport code

As of the removal of 'Legacy' mode, there can no longer be a difference between the transport you advertise in your membership and the transport you publish media on, so the local transport code can be simplified considerably. For instance, the function which gets the local transport can simply return a promise rather than being reactive.

I kept the local transport as an Observable in other modules so that they could easily be tested with existing marble tests.

src/state/CallViewModel/CallViewModelTestUtils.ts

@@ -210,11 +210,8 @@ export function withCallViewModel(mode: MatrixRTCMode) {
         connectionState$,
         windowSize$,
         localTransport: {
-          active$: constant({
-            transport: exampleTransport,
-            sfuConfig: exampleSfuConfig,
-          }),
-          advertised$: constant(exampleTransport),
+          transport: exampleTransport,
+          sfuConfig: exampleSfuConfig,
         },
         connectionFactory: {
           createConnection(