Simplify code paths for building to-device MatrixEvents (#4729)

We don't need all the complicated stuff when we have a to-device event, so
let's simplify.

src/event-mapper.ts

@@ -25,7 +25,8 @@ export interface MapperOpts {
     preventReEmit?: boolean;
     // decrypt event proactively
     decrypt?: boolean;
-    // the event is a to_device event
+
+    /** @deprecated no longer used */
     toDevice?: boolean;
 }
 
@@ -34,10 +35,6 @@ export function eventMapperFor(client: MatrixClient, options: MapperOpts): Event
     const decrypt = options.decrypt !== false;
 
     function mapper(plainOldJsObject: Partial<IEvent>): MatrixEvent {
-        if (options.toDevice) {
-            delete plainOldJsObject.room_id;
-        }
-
         const room = client.getRoom(plainOldJsObject.room_id);
 
         let event: MatrixEvent | undefined;
@@ -74,9 +71,6 @@ export function eventMapperFor(client: MatrixClient, options: MapperOpts): Event
             event.setThread(thread);
         }
 
-        // TODO: once we get rid of the old libolm-backed crypto, we can restrict this to room events (rather than
-        //   to-device events), because the rust implementation decrypts to-device messages at a higher level.
-        //   Generally we probably want to use a different eventMapper implementation for to-device events because
         if (event.isEncrypted()) {
             if (!preventReEmit) {
                 client.reEmitter.reEmit(event, [MatrixEventEvent.Decrypted]);