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/localMember/LocalMember.test.ts

@@ -230,6 +230,7 @@ describe("LocalMembership", () => {
     },
     roomId: "!test-room-id:example.org",
     baseUrl: "https://matrix.example.org",
+    matrixRTCMode: MATRIX_RTC_MODE,
   };
 
   beforeEach(() => {