Remove ci_send_deployment_hook_when_start feature flag

Enable deployment started webhook and chat notifications

app/models/deployment.rb

@@ -79,8 +79,6 @@ class Deployment < ApplicationRecord
 
     after_transition any => :running do |deployment|
       deployment.run_after_commit do
-        next unless Feature.enabled?(:ci_send_deployment_hook_when_start, deployment.project)
-
         Deployments::ExecuteHooksWorker.perform_async(id)
       end
     end

config/feature_flags/development/ci_send_deployment_hook_when_start.yml

----
-name: ci_send_deployment_hook_when_start
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/41214
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/247137
-group: group::progressive delivery
-type: development
-default_enabled: false

spec/models/deployment_spec.rb

@@ -114,14 +114,6 @@ RSpec.describe Deployment do
         deployment.run!
       end
 
-      it 'does not execute Deployments::ExecuteHooksWorker when feature is disabled' do
-        stub_feature_flags(ci_send_deployment_hook_when_start: false)
-        expect(Deployments::ExecuteHooksWorker)
-            .not_to receive(:perform_async).with(deployment.id)
-
-        deployment.run!
-      end
-
       it 'executes Deployments::DropOlderDeploymentsWorker asynchronously' do
         expect(Deployments::DropOlderDeploymentsWorker)
             .to receive(:perform_async).once.with(deployment.id)