Account deactivation is not working if logged in using SSO

[bmarty]

Step to reproduce:

• Connect to mozilla.modular.im, using SSO (for instance)
• Try to deactivate your account with any password (because you do not have password for your matrix account since its SSO) (settings/general/deactivate account)

This request is done:
/_matrix/client/r0/account/deactivate
with body

{"auth":{"type":"m.login.password","user":"@alice:mozilla.org","identifier":{"type":"m.id.user","user":"@alice:mozilla.org"},"password":"leakPassword"},"erase":false}

and get a 400 with body:

{"errcode":"M_UNKNOWN","error":"Password login has been disabled."}

UI Side, we got this:

I think the deactivation of the account should be disabled following the change password capability of the home server seems it's a bit related.

But we maybe should propose a way to deactivate the account anyway and to wipe out data if the user wants it to conform to GDPR directives.

[t3chguy]

Mozilla's Synapse doesn't yet support SSO for UIA.
matrix-org/synapse#5667 (comment) (waiting on matrix-org/synapse#6705)
Duplicate of #10847

[bmarty]

Also #12692 is a thing