Remove enable_cluster_application_crossplane FF

Remove this feature flag as the application is proven to install without
issue since 12.5.

app/assets/javascripts/clusters/components/applications.vue

@@ -129,9 +129,6 @@ export default {
     crossplaneInstalled() {
       return this.applications.crossplane.status === APPLICATION_STATUS.INSTALLED;
     },
-    enableClusterApplicationCrossplane() {
-      return gon.features && gon.features.enableClusterApplicationCrossplane;
-    },
     enableClusterApplicationElasticStack() {
       return gon.features && gon.features.enableClusterApplicationElasticStack;
     },
@@ -519,7 +516,6 @@ Crossplane runs inside your Kubernetes cluster and supports secure connectivity
         </div>
       </application-row>
       <application-row
-        v-if="enableClusterApplicationCrossplane"
         id="crossplane"
         :logo-url="crossplaneLogo"
         :title="applications.crossplane.title"

app/controllers/clusters/clusters_controller.rb

@@ -14,7 +14,6 @@ class Clusters::ClustersController < Clusters::BaseController
   before_action :update_applications_status, only: [:cluster_status]
   before_action only: [:show] do
     push_frontend_feature_flag(:enable_cluster_application_elastic_stack)
-    push_frontend_feature_flag(:enable_cluster_application_crossplane)
   end
 
   helper_method :token_in_session

app/services/clusters/applications/base_service.rb

@@ -68,7 +68,7 @@ module Clusters
       end
 
       def invalid_application?
-        unknown_application? || (application_name == Applications::ElasticStack.application_name && !Feature.enabled?(:enable_cluster_application_elastic_stack)) || (application_name == Applications::Crossplane.application_name && !Feature.enabled?(:enable_cluster_application_crossplane))
+        unknown_application? || (application_name == Applications::ElasticStack.application_name && !Feature.enabled?(:enable_cluster_application_elastic_stack))
       end
 
       def unknown_application?

changelogs/unreleased/remove_enable_cluster_application_crossplane_flag.yml

+---
+title: Enable ability to install Crossplane app by default
+merge_request: 22141
+author:
+type: changed

doc/user/clusters/applications.md

@@ -420,18 +420,6 @@ install Crossplane using the
 [`values.yaml`](https://github.com/crossplaneio/crossplane/blob/master/cluster/charts/crossplane/values.yaml.tmpl)
 file.
 
-#### Enabling installation
-
-This is a preliminary release of Crossplane as a GitLab-managed application. By default,
-the ability to install it is disabled.
-
-To allow installation of Crossplane as a GitLab-managed application, ask a GitLab
-administrator to run following command within a Rails console:
-
-```ruby
-Feature.enable(:enable_cluster_application_crossplane)
-```
-
 ### Elastic Stack
 
 > Introduced in GitLab 12.7 for project- and group-level clusters.

spec/frontend/clusters/components/applications_spec.js

@@ -17,7 +17,6 @@ describe('Applications', () => {
 
     gon.features = gon.features || {};
     gon.features.enableClusterApplicationElasticStack = true;
-    gon.features.enableClusterApplicationCrossplane = true;
   });
 
   afterEach(() => {