fix: Don't depend on home_server field in registration response

This field is deprecated in the client-server API and has been removed from
Dendrite, causing the UI to error when handling a registration response.

Reference: https://spec.matrix.org/v1.6/client-server-api/#post_matrixclientv3register

It appears that the `home_server` field wasn't actually being used here
anyhow, so just remove the line that tries to fetch it. The spec indicates
that if it is needed, the caller should instead split the user_id at the
first colon.

Fixes zeratax#103

matrix_registration/api.py

@@ -260,7 +260,6 @@ def create_account_from_form(form):
     logger.debug("account creation succeded!")
     return jsonify(
         access_token=account_data["access_token"],
-        home_server=account_data["home_server"],
         user_id=account_data["user_id"],
         status="success",
         status_code=200,