chore: remove httpHeaders from server handlers (#28011)

it was kind of a hack in the first place, and now it's not used at all

src/legacy/core_plugins/interpreter/server/lib/__tests__/create_handlers.js

@@ -75,7 +75,6 @@ describe('server createHandlers', () => {
   it('provides helper methods and properties', () => {
     expect(handlers).to.have.property('environment', 'server');
     expect(handlers).to.have.property('serverUri');
-    expect(handlers).to.have.property('httpHeaders', mockRequest.headers);
     expect(handlers).to.have.property('elasticsearchClient');
   });
 

src/legacy/core_plugins/interpreter/server/lib/create_handlers.js

@@ -31,7 +31,6 @@ export const createHandlers = (request, server) => {
       config.has('server.rewriteBasePath') && config.get('server.rewriteBasePath')
         ? `${server.info.uri}${config.get('server.basePath')}`
         : server.info.uri,
-    httpHeaders: request.headers,
     elasticsearchClient: async (...args) => {
       // check if the session is valid because continuing to use it
       if (isSecurityEnabled(server)) {