Removes references to the legacy namespace dropdown

What does this MR do and why?

As part of New project screen is unusable if user is in a ... (#26732 - closed) it looks like we removed the legacy js-select-namespace component from the new project page.

This MR removes the remaining code referencing the old dropdown. I noticed it wasnt being used while working on Replace jQuery in ee/custom_project_templates.j... (!89355 - merged)

Screenshots or screen recordings

No visual changes - All tests passing

MR acceptance checklist

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Related to #362998 (closed)