Remove phabricator as import source on database

What does this MR do and why?

Phabricator is no longer supported and has been removed as a possible import source from the codebase in !117649 (merged).

This adds a migration to remove Phabricator as an import source from the database table for application settings.

main: == 20230419105225 RemovePhabricatorFromApplicationSettings: migrating =========
main: == 20230419105225 RemovePhabricatorFromApplicationSettings: migrated (0.3317s)

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #388245 (closed)