Backport #33325 by CrimsonEdgeHope
Fix password form missing whilst linking account even with
`ENABLE_PASSWORD_SIGNIN_FORM = true`.
Remove redundant empty box in account linking sign up page when
`LinkAccountMode` is true.
Co-authored-by: CrimsonEdgeHope <92579614+CrimsonEdgeHope@users.noreply.github.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>