Move PR perf benchmark pipeline out of PR directory

Author: gbanasiak

.buildkite/pipelines/pull-request-performance-benchmark.yml

@@ -0,0 +1,3 @@
+steps:
+  - label: ":pipeline: Generate steps"
+    command: bash .buildkite/scripts/generate-pr-performance-benchmark.sh | buildkite-agent pipeline upload

.buildkite/pipelines/pull-request/performance-benchmark.yml

@@ -323,7 +323,7 @@ spec:
       name: elasticsearch / pull-request / performance-benchmark
     spec:
       repository: elastic/elasticsearch
-      pipeline_file: .buildkite/pipelines/pull-request/performance-benchmark.yml
+      pipeline_file: .buildkite/pipelines/pull-request-performance-benchmark.yml
       env:
         ELASTIC_PR_COMMENTS_ENABLED: "true"
         ELASTIC_SLACK_NOTIFICATIONS_ENABLED: "true"