kubernetes-backend: remove unused APPLICATION_JSON constant

The constant was exported but never referenced in production code.
The only usage was in the test file, where it's now inlined.

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
Made-with: Cursor

plugins/kubernetes-backend/src/service/KubernetesProxy.test.ts

@@ -43,7 +43,6 @@ import {
   KubernetesCredential,
 } from '@backstage/plugin-kubernetes-node';
 import {
-  APPLICATION_JSON,
   HEADER_KUBERNETES_AUTH,
   HEADER_KUBERNETES_CLUSTER,
   KubernetesProxy,
@@ -95,7 +94,7 @@ describe('KubernetesProxy', () => {
       header: jest.fn((key: string) => {
         switch (key) {
           case 'Content-Type': {
-            return APPLICATION_JSON;
+            return 'application/json';
           }
           case HEADER_KUBERNETES_CLUSTER: {
             return clusterName;

plugins/kubernetes-backend/src/service/KubernetesProxy.ts

@@ -46,8 +46,6 @@ import {
   PermissionsService,
 } from '@backstage/backend-plugin-api';
 
-export const APPLICATION_JSON: string = 'application/json';
-
 /**
  * The header that is used to specify the cluster name.
  *