ci: drop mac-x64 test leg (Intel macOS hardware retired)

The macos-15 (x64) agent hardware no longer exists, so remove the mac-x64
matrix entry. macOS is now tested on mac-aarch64 only. The darwin-x86-64
.dylib is still produced by the release GitHub Action, and build_native.yaml
keeps its uname-based arch detection so an x64 macOS runner would still build
correctly if ever reintroduced.

Author: bluestreak01

ci/run_tests_pipeline.yaml

@@ -54,10 +54,6 @@ stages:
               imageName: "macos-15-arm64"
               poolName: "Azure Pipelines"
               jdkArch: "arm64"
-            mac-x64:
-              imageName: "macos-15"
-              poolName: "Azure Pipelines"
-              jdkArch: "x64"
             windows-msvc-2022-x64:
               imageName: "windows-2022"
               poolName: "Azure Pipelines"