Remove IS details from requestToken to HS
This removes the IS details (server and access token) from `requestToken` calls
to the HS, as long as the HS supports the new separate add and bind mode. In
this mode, all of the 3PID validation is handled by the HS, so the IS details
are not used.
Fixes https://github.com/vector-im/riot-web/issues/10933
