Remove unused cviebrock/eloquent-sluggable dependency (#1109)

`cviebrock/eloquent-sluggable` was installed while fixing a Kubernetes client version bump[1] but isn't actually used. I assume this was done by mistake - wrangling composer dependencies can be a challenge!

[1]: https://github.com/wbstack/api/pull/479/changes#diff-d2ab9925cad7eac58e0ff4cc0d251a937ecf49e4b6bf57f8b95aab76648a9d34R12

Bug: T424468

Author: outdooracorn

composer.json

@@ -9,7 +9,6 @@
     "type": "project",
     "require": {
         "absszero/laravel-stackdriver-error-reporting": "^1.7",
-        "cviebrock/eloquent-sluggable": "^10.0",
         "doctrine/dbal": "^3.1",
         "firebase/php-jwt": "^6.10",
         "google/recaptcha": "^1.2",