create_user: Remove assert about needing prereg_user if realm_creation.
create_user: Remove assert about needing prereg_user if realm_creation.

Closes #22274.

This assertion was added in 4b903c5dcdf3ea236a289f6d204b2953d11445c7
where it may have made sense, because indeed when doing realm creation
there was always a PreregistrationUser (created because realms were
created via going to a generated realm creation link). With the addition
of the create_realm command that's no longer the case.

It would be unnatural to create a PreregistrationUser in the
realm_creation command, because there is no confirmation link for it to
be tied to - and it just doesn't make sense conceptually.

The intended, correct behavior added in
4b903c5dcdf3ea236a289f6d204b2953d11445c7 is still maintained - the code
lower down correctly handles the
(prereg_user is None and realm_creation) case.

rht/zulip,zulip/zulip,rht/zulip,zulip/zulip,zulip/zulip,andersk/zulip,andersk/zulip,zulip/zulip,rht/zulip,andersk/zulip,andersk/zulip,zulip/zulip,rht/zulip,zulip/zulip,andersk/zulip,rht/zulip,zulip/zulip,rht/zulip,rht/zulip,andersk/zulip,andersk/zulip