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/room/InCallView.tsx

@@ -135,7 +135,9 @@ export const ActiveCall: FC<ActiveCallProps> = (props) => {
         encryptionSystem: props.e2eeSystem,
         autoLeaveWhenOthersLeft,
         waitForCallPickup: waitForCallPickup && sendNotificationType === "ring",
-        matrixRTCMode$: matrixRTCModeSetting.value$,
+        // We merely sample the current mode here, so the user would need to
+        // manually rejoin to switch to a different one.
+        matrixRTCMode: matrixRTCModeSetting.value$.value,
       },
       reactionsReader.raisedHands$,
       reactionsReader.reactions$,