Remove Windows BWC pull request pipeline (#110664)

We've already removed Windows-specific BWC jobs in our periodic
pipelines. They shouldn't behave differently and are very prone to
timeouts so let's just remove them from pull requests when the
`test-windows` label is added.

.buildkite/pipelines/pull-request/bwc-snapshots-windows.yml

@@ -1,20 +0,0 @@
-config:
-  allow-labels: test-windows
-steps:
-  - group: bwc-snapshots-windows
-    steps:
-      - label: "{{matrix.BWC_VERSION}} / bwc-snapshots-windows"
-        key: "bwc-snapshots-windows"
-        command: .\.buildkite\scripts\run-script.ps1 bash .buildkite/scripts/windows-run-gradle.sh
-        env:
-          GRADLE_TASK: "v{{matrix.BWC_VERSION}}#bwcTest"
-        timeout_in_minutes: 300
-        matrix:
-          setup:
-            BWC_VERSION: $SNAPSHOT_BWC_VERSIONS
-        agents:
-          provider: gcp
-          image: family/elasticsearch-windows-2022
-          machineType: custom-32-98304
-          diskType: pd-ssd
-          diskSizeGb: 350