catalog/externalcatalog: delete namespace entries during clean up

[fqazi]

Previously, the external catalog logic would never clean up namespace entries for descriptors when DropIngestedExternalCatalog was invoked. This would lead to left over junk in the system.namespace table, contrary to the normal object deletion protocol. To address this, this patch also deletes namespace entries.

Fixes: #140551

Release note: None

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

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[cockroach-teamcity]

This change is 

[msbutler]

oof. Nice find!

[msbutler]

borsing asap to reduce flakiness

bors r+

[fqazi]

@msbutler TFTR

bors r+

[craig]

Already running a review

[craig]

Build succeeded:

• local_roachtest_fips
• windows_build
• lint
• acceptance
• macos_arm64_build
• docker_image_amd64
• linux_amd64_fips_build
• examples_orms
• unit_tests
• linux_amd64_build
• linux_arm64_build
• check_generated_code
• local_roachtest
• macos_amd64_build

[blathers-crl]

Based on the specified backports for this PR, I applied new labels to the following linked issue(s). Please adjust the labels as needed to match the branches actually affected by the issue(s), including adding any known older branches.

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Issue #140551: branch-release-25.1.

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[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error setting reviewers, but backport branch blathers/backport-release-25.1-140564 is ready: POST https://api.github.com/repos/cockroachdb/cockroach/pulls/140594/requested_reviewers: 422 Reviews may only be requested from collaborators. One or more of the teams you specified is not a collaborator of the cockroachdb/cockroach repository. []

Backport to branch 25.1.x failed. See errors above.

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