Fix project page 404 after rename and removing new name from alias slugs

Summary:
When renaming a project to a slug already listed under Additional hashtags and explicitly also removing that to-become slug, accessing the project via the URL /tag/projectname/ returned a 404 until someone added the current project name explicitly under "Additional hashtags" again.

In that case, do not remove the alternative hashtag to avoid the 404.

Closes T15636

Test Plan: See steps in https://we.phorge.it/T15636

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15636

Differential Revision: https://we.phorge.it/D25453