Removes all the irrelevant import related code and columns

What does this MR do?

CE counterpart: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21497

This MR focuses on cleaning up all the logic that was implemented in order to make the migration of the mirror logic from the project table over to the project_import_state table.

Architectural questions:

• Should we keep the convenience methods in the Project and delegate them over to the ProjectImportState? Or should we always use project.import_state.jid instead of project.import_state_jid for example

• Check the GithubImport and LegacyGithubImport logic since they both make use of Project#ensure_import_state which will be removed in this MR

• Some of the queries could be better tuned now that all the columns were successfully migrated over to the ProjectImportState table, we should explore those improvements.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?