[16.10] Backport Delete callback should use namespace_id

What does this MR do and why?

Backport !147459 (merged) to %16.10

From the original MR:

Search::Zoekt::Index delete_from_index callback should use namespace_id (not null) since zoekt_enabled_namespace_id foreign key has ON DELETE SET NULL

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).
• Set the milestone of the merge request to match the target backport branch version.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:package-and-test-ee job has either succeeded or been approved by a Software Engineer in Test.

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