make "Import your data" screen optional with upstream IdP

[matrixbot]

This issue was originally created by @pierreozoux at matrix-org/matrix-authentication-service#3170.

Hi,

we use an upstream IdP that is well configured.
It maps well the localpart, the email and the name.

Yet there is a confirmation screen:

I checked in the options, but didn't find a way to deactivate it.

Thanks for your help!