Don't ask about "only user" when importing a shared secret team key

When someone imports a shared secret team key (i.e. a key flagged as
group key with secret subkeys and without secret primary key) then their
answer to the question whether they are the only user of this key will
very likely be No. Hence, in this case, we omit asking this question.

We still ask the question if a group key with secret primary key is
imported because the person who controls the team key still may want to
mark the key as ultimately trusted.

• GnuPG-bug-id: T8098