✂ Remove lib/importer and corresponding spec/lib/importer

Per https://github.com/samvera/hyku/releases/tag/v5.0.0, these importers
have been deprecated.

The "Commit History of Directories" shows that
there as been little activity, the recent activity being appeasing
Rubocop and working through upgrades.  Further, these files are
conceptually subsumed by Bulkrax.

<details><summary>Commit History of Directories</summary>

```bash
❯ git slog lib/importer
* 6a48652 — Hyrax 5 upgrade rubocop fixes & get specs running in CI (#2065) LaRita Robinson, (2023-12-16)
* 59626ef — 🧹 Rubocop'd lengths and other low hanging fruit Kirk Wang, (2023-12-13)
* fecb4e9 — 🧹 Ran `bundle exec rubocop -a` Kirk Wang, (2023-12-13)
* 0a6d047 — Upgrade to Hyrax 3.4.0 (#1811) Rob Kaufman, (2022-07-07)
* edafd85 — Rubocop fixes (#1640) Rob Kaufman, (2020-08-04)
* 9326387 — update to hyrax 2.2 (#1543) Rob Kaufman, (2018-10-04)
*   7526936 — Merge pull request #1541 from ubiquitypress/hyku-csv-importer Rob Kaufman, (2018-08-21)
|\
| * 904a369 — Remove :files from attributes Remove method to replace existing file_set Claire, (2018-07-20) (tag: v1.0.0)
| * 1cf7f5c — WIP- set uploaded_files param to use existing Actor Claire, (2018-07-19)
| * 9650e54 — Restore callbacks Claire, (2018-07-12)
| * e6a358a — Coerce UploadedFile to an array and use upstream Job Claire, (2018-07-11)
| * 93cb695 — Import files from CSV Claire, (2018-07-06)
* | bc51174 — Hyrax 2.1 upgrade (#1529) Rob Kaufman, (2018-07-26)
|/
* 116d4b9 — Upgrade Hyrax to 2.0.2 and upgrade gems with security vulnerabilities (#1519) Julie Allinson, (2018-03-22)
* d85952b — Changes necessary to get simple CSV import to run Michael J. Giarlo, (2017-08-22)
* a4024f4 — Make ObjectFactory rubocop-clean Joe Atzberger, (2017-08-22)
* 7618c3a — Refactor csv_importer and improve test accuracy Joe Atzberger, (2017-08-22)
* 21eb1e6 — Add a note about a broken method Justin Coyne, (2017-06-15)
* 86eedaa — Update Hyrax and other dependencies Justin Coyne, (2017-04-13)
* bfecea2 — Refactor importers to use Hyrax's actor stack Justin Coyne, (2017-02-23)
* 554b8a8 — Pass ability to the actor Justin Coyne, (2017-02-17)
* a8b25cd — Point at the Hyrax repo Justin Coyne, (2016-12-03)
* 5d005c9 — [WIP] Hyrax! Justin Coyne, (2016-12-02)
* fe229ca — Rename Lerna to Hyku. Fixes #461 Justin Coyne, (2016-11-07)
* 1f5fe23 — Update dependencies, Sufia to 7.0.0.rc1 Justin Coyne, (2016-07-12)
* d2c8fb1 — Rename hybox to lerna Michael J. Giarlo, (2016-06-08)
* 5de0440 — Add a CSV importer Justin Coyne, (2016-05-23)
* 5dcbb9b — Use User.batch_user instead of the deprecated batchuser Justin Coyne, (2016-05-20)
* d2a6c3c — Update to latest rubocop and rubocop-rspec Chris Beer, (2016-05-20)
* 642df79 — MODS/PURL importer Justin Coyne, (2016-05-19)
```

```bash
❯ git slog spec/lib/importer/
* 1c04b9e — 🧹 Hyrax 5 upgrade additional specs (#2067) LaRita Robinson, (2023-12-18)
* 6a48652 — Hyrax 5 upgrade rubocop fixes & get specs running in CI (#2065) LaRita Robinson, (2023-12-16)
* fecb4e9 — 🧹 Ran `bundle exec rubocop -a` Kirk Wang, (2023-12-13)
* 0a6d047 — Upgrade to Hyrax 3.4.0 (#1811) Rob Kaufman, (2022-07-07)
* edafd85 — Rubocop fixes (#1640) Rob Kaufman, (2020-08-04)
* f9a3a64 — Use paths based on Rails.root instead of pure relative paths Chris Colvard, (2020-07-08)
* 1b3c5ed — rubocop fixes nicholas, (2018-10-12)
* aeb9904 — update FactoryGirl to FactoryBot nicholas, (2018-10-09)
* 93cb695 — Import files from CSV Claire, (2018-07-06)
* 116d4b9 — Upgrade Hyrax to 2.0.2 and upgrade gems with security vulnerabilities (#1519) Julie Allinson, (2018-03-22)
* 7618c3a — Refactor csv_importer and improve test accuracy Joe Atzberger, (2017-08-22)
* 86eedaa — Update Hyrax and other dependencies Justin Coyne, (2017-04-13)
* bfecea2 — Refactor importers to use Hyrax's actor stack Justin Coyne, (2017-02-23)
* 8ecb861 — Require rails_helper in one place instead of 100 Joe Atzberger, (2017-02-14)
* 7498ecb — Update to latest hyrax Justin Coyne, (2016-12-20)
* a8b25cd — Point at the Hyrax repo Justin Coyne, (2016-12-03)
* 5d005c9 — [WIP] Hyrax! Justin Coyne, (2016-12-02)
* 5de0440 — Add a CSV importer Justin Coyne, (2016-05-23)
* d2a6c3c — Update to latest rubocop and rubocop-rspec Chris Beer, (2016-05-20)
* 642df79 — MODS/PURL importer Justin Coyne, (2016-05-19)
```

</details>