[SDK] matrix-android-sdk2 uses an unspecced `original_event` field from the `/relations` response

### Description

The [`/relations` endpoint](https://spec.matrix.org/v1.4/client-server-api/#get_matrixclientv1roomsroomidrelationseventid) does not return an `original_event` field, yet the [matrix-android-sdk2 uses it](https://github.com/matrix-org/matrix-android-sdk2/blob/a4a039b04332746f9592d821512fbc87d11e7ee1/matrix-sdk-android/src/main/java/org/matrix/android/sdk/internal/session/room/relation/RelationsResponse.kt#L25).

We would like to remove support for this from Synapse in < 6 months. See matrix-org/synapse#12930 for more context.