Remove all the relevant "import" columns that were migrated over to ProjectImportState from the project table

After https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5522 was merged there were a few columns left in the Project table that were used as fallback in the event that the import specific settings were not already migrated into their own table.

The columns are:

• import_jid
• import_status
• mirror_last_update_at
• mirror_last_update_started_at
• import_error

There were also methods introduced which ensured that the ProjectImportState table was created such as:

• human_import_status_name
• import_schedule
• force_import_start
• import_start
• import_finish
• import_fail
• import_jid=
• import_jid
• import_error=
• import_error
• import_status=
• import_status

We should ensure that all projects with import data were successfully migrated and remove and cleanup the columns and methods listed above respectively

\cc @DouweM