Bug: Placeholder user appears in assign dropdowns

Problem

Dropdowns often include the author of the issue, MR, epic, etc..

This causes the placeholder user to appear at the top of the dropdown list that appears when assigning something.

We would like the placeholder not to appear in those dropdowns.

Steps

1. Do a Direct Transfer import with user mapping enabled (enable importer_user_mapping and bulk_import_importer_user_mapping feature flags)
2. On an imported issue, click the assign widget, or @ in the comments, to see the dropdowns.

Screen_Recording_2024-09-04_at_4.15.29_PM