[Backend] Remove unused type import

Validator types are used with sequelize-typescript, but they are imported at least by sequelize.
No explicit import is needed.

Author: santeri0200

services/backend/package.json

@@ -39,7 +39,6 @@
     "@types/lodash": "^4.17.17",
     "@types/morgan": "^1.9.10",
     "@types/node": "^22.15.31",
-    "@types/validator": "^13.15.1",
     "tsx": "^4.19.4",
     "typescript": "^5.9.2"
   },