Fix flaky spec for invite members for tasks

What does this MR do and why?

Because assigning multiple users to issues is not possible on EE, a specific assignee cannot be guaranteed. Removing the check for that here.

Note that multiple assignments for EE are tested here: https://gitlab.com/gitlab-org/gitlab/blob/master/ee/spec/services/ee/members/create_service_spec.rb#L119

This fixes #343792 (closed)

Introduced here: !69299 (comment 714087858)

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