Blank descriptor wallets currently do not have HD seeds and none can be added (or ‘set’) by the user, so remove the reference in the tooltip.
As I understand it, descriptor wallets don’t have a global HD seed and don’t store the HD seeds for keys they generate. Currently, no new HD seeds can be added by the user (even for old wallets since sethdseed was removed), though it may be possible in the future, eg - https://github.com/bitcoin/bitcoin/pull/33043