[7.17] Clean up pipelines (elastic#187490)

## Summary
Removes unused pipelines, and their related scripts: 
 - package_testing.yml
 - demo_env.yml

Updates a pipeline to the new buildkite agent targeting format:
 - performance/daily.yml (missed backport after migration)

.buildkite/pipelines/performance/daily.yml

@@ -11,20 +11,29 @@ steps:
   - label: ':male-mechanic::skin-tone-2: Pre-Build'
     command: .buildkite/scripts/lifecycle/pre_build.sh
     agents:
-      queue: kibana-default
+      image: family/kibana-ubuntu-2004
+      imageProject: elastic-images-prod
+      provider: gcp
+      machineType: n2-standard-2
 
   - wait
 
   - label: ':factory_worker: Build Kibana Distribution and Plugins'
     command: .buildkite/scripts/steps/build_kibana.sh
     agents:
-      queue: c2-16
+      image: family/kibana-ubuntu-2004
+      imageProject: elastic-images-prod
+      provider: gcp
+      machineType: c2-standard-16
     key: build
 
   - label: ':muscle: Performance Tests with Playwright config'
     command: .buildkite/scripts/steps/functional/performance_playwright.sh
     agents:
-      queue: c2-16
+      image: family/kibana-ubuntu-2004
+      imageProject: elastic-images-prod
+      provider: gcp
+      machineType: c2-standard-16
     depends_on: build
 
   - wait: ~
@@ -33,4 +42,7 @@ steps:
   - label: ':male_superhero::skin-tone-2: Post-Build'
     command: .buildkite/scripts/lifecycle/post_build.sh
     agents:
-      queue: kibana-default
+      image: family/kibana-ubuntu-2004
+      imageProject: elastic-images-prod
+      provider: gcp
+      machineType: n2-standard-2