ui: remove old nspace argument from the proxy instance repository (#10039)

The extra argument meant that the blocking query configuration wasn't
being read properly, and therefore the correct ?index wasn't being sent
with the request.

.changelog/10039.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+ui: ensure proxy instance API requests perform blocking queries correctly
+```

ui/packages/consul-ui/app/services/repository/proxy.js

@@ -36,7 +36,7 @@ export default class ProxyService extends RepositoryService {
   }
 
   @dataSource('/:ns/:dc/proxy-instance/:serviceId/:node/:id')
-  findInstanceBySlug(params, nspace, configuration) {
+  findInstanceBySlug(params, configuration) {
     return this.findAllBySlug(params, configuration).then(function(items) {
       let res = {};
       if (get(items, 'length') > 0) {