Do not run NAOT arm64 OSX testing on all PRs

These legs have been constantly timing out since beginning of January because Helix is overloaded (e.g. #112073). In looking at why we don't see timeouts with CoreCLR, I found out we do not test ARM64 OSX with CoreCLR on all PRs. So this moves it to the same plan as e.g. ARM32 Linux.

Author: MichalStrehovsky

eng/pipelines/runtime.yml

@@ -609,7 +609,6 @@ extends:
           - osx_x64
           - linux_arm64
           - windows_arm64
-          - osx_arm64
           variables:
           - name: timeoutPerTestInMinutes
             value: 60