Update global.json

Remove unused SDKs

Author: CZEMacLeod

global.json

@@ -5,8 +5,6 @@
     "allowPrerelease": true
   },
   "msbuild-sdks": {
-    "Microsoft.Build.NoTargets": "2.0.1",
-    "MSBuild.SDK.SystemWeb": "4.0.88",
-    "MSBuild.SDK.SystemWeb.RazorLibrary": "4.0.82"
+    "Microsoft.Build.NoTargets": "2.0.1"
   }
-}
+}