AO3-6664 Imported_from_url search is too slow

[ariana-paris]

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Issue

https://otwarchive.atlassian.net/browse/AO3-6664

Purpose

Removes a very slow partial match query when searching for works by imported_from_url.

Testing Instructions

See Jira ticket

References

Are there other relevant issues/pull requests/mailing list discussions?

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