[AUDIT] Run knip over twenty-server (#21159)

# Introduction
Run [knip](https://knip.dev/) over twenty-server
Used config:
```json
{
  "$schema": "https://unpkg.com/knip@5/schema.json",
  "workspaces": {
    "packages/twenty-server": {
      "entry": [
        "src/main.ts",
        "src/command/command.ts",
        "src/queue-worker/queue-worker.ts",
        "src/database/scripts/setup-db.ts",
        "src/database/scripts/truncate-db.ts",
        "src/database/clickHouse/migrations/run-migrations.ts",
        "src/database/clickHouse/seeds/run-seeds.ts",
        "src/instrument.ts",
        "lingui.config.ts",
        "test/integration/graphql/codegen/index.ts",
        "test/integration/utils/setup-test.ts",
        "test/integration/utils/teardown-test.ts",
        "scripts/**/*.ts",
        "**/*.spec.ts",
        "**/*.integration-spec.ts"
      ],
      "project": ["src/**/*.ts", "test/**/*.ts", "scripts/**/*.ts"],
      "ignore": [
        "src/database/typeorm/**/migrations/**",
        "src/database/typeorm/**/*.entity.ts",
        "**/*.workspace-entity.ts",
        "**/logic-function-resource/constants/seed-project/**"
      ],
      "ignoreDependencies": ["@types/psl", "@types/aws-lambda"],
      "ignoreBinaries": ["nest", "lingui", "typeorm"]
    }
  }
}
```

packages/twenty-server/src/engine/metadata-modules/flat-application-variable/constants/flat-application-variable-editable-properties.constant.ts

@@ -1,7 +0,0 @@
-import { type MetadataEntityPropertyName } from 'src/engine/metadata-modules/flat-entity/constant/all-entity-properties-configuration-by-metadata-name.constant';
-
-export const FLAT_APPLICATION_VARIABLE_EDITABLE_PROPERTIES = [
-  'key',
-  'description',
-  'isSecret',
-] as const satisfies MetadataEntityPropertyName<'applicationVariable'>[];