Remove win-arm RID from PortableRuntimeIdentifierGraph.json (#116832)

The `win-arm` RID was present in the portable runtime identifier graph but is not supported with critical assets like AppHost. This caused a poor user experience where developers attempting to target win-arm would receive confusing error messages about missing application hosts rather than a clear indication that the RID is not supported.

This change removes the win-arm RID entry from PortableRuntimeIdentifierGraph.json, which aligns with the fact that we dropped support for this RID and don't ship the necessary assets for it. Users should now receive a `NETSDK1083` error indicating that the RID is not supported.

Author: Copilot

src/libraries/Microsoft.NETCore.Platforms/src/PortableRuntimeIdentifierGraph.json

@@ -457,11 +457,6 @@
           "any"
         ]
       },
-      "win-arm": {
-        "#import": [
-          "win"
-        ]
-      },
       "win-arm64": {
         "#import": [
           "win"