Stop setting IS input field on account change
This stops setting a value in the IS input on account change. While it may have
been marginally useful if you have the form open and change on a different
device, it also seems to pick up changes on the current device, leading to
strange UX locally.
Fixes https://github.com/vector-im/riot-web/issues/10756
Fixes https://github.com/vector-im/riot-web/issues/10757
