make sure setDefaultKeyId resolves even if no change is made - #5485

For the archaeological record:

setDefaultKeyId has had a listener on ClientEvent.AccountData to wait for the remote echo ever since 4S was introduced in #832. That was fine at that point, because MatrixClient.setAccountData didn't have any logic to filter out no-op changes, so there would reliably be a remote echo.

Later, #4695 changed MatrixClient.setAccountData so that (a) it did nothing if the change was a no-op; (b) it had its own listener on ClientEvent.AccountData to wait for the remote echo. [Aside: the comments added in that change also assert that there will be no remote echo in the case of a no-op change of account data; it is unclear to me now if that is accurate.] That change made the listener in setDefaultKeyId both redundant and unreliable (since in the case of changes that setAccountData deemed a no-op, there would be no remote echo).

Nevertheless, setDefaultKeyId continued to work ok in the specific case of removing the default key ID (i.e. setting the account data to {}), because, until https://github.com/matrix-org/matrix-js-sdk/pull/5446/changes#diff-7237d115811d917dbc234402ea3e85ced2a568c07005f45344d3a513f30cf68cL295-L301, an account data value of {} was special-cased, so repeated calls to MatrixClient.setAccountData with a value of {} would make an API hit each time.

In other words, between #4695 and #5446, it was working ok, but for all the wrong reasons.

This PR completes the work that should have been done in #4695, by removing the now-redundant listener from setDefaultKeyId.