Base JWT service endpoint choice directly on the MatrixRTC mode

Since we removed the extra 'legacy' mode, meaning there's nothing simplified anymore by translating things over to a JWT service version.

(Also, simplify some things by making the mode static for the duration of a call.)

src/state/CallViewModel/CallViewModelTestUtils.ts

@@ -238,7 +238,7 @@ export function withCallViewModel(mode: MatrixRTCMode) {
             );
           },
         },
-        matrixRTCMode$: constant(mode),
+        matrixRTCMode: mode,
         ...options,
       },
       raisedHands$,