Remove Microsoft.DotNet.PackageValidation from global.json (#56972)

I incorrectly added it back while resolving merge conflicts in #56719 but it was removed from global.json with #56712

Author: akoeplinger

global.json

@@ -13,7 +13,6 @@
   },
   "msbuild-sdks": {
     "Microsoft.DotNet.Build.Tasks.TargetFramework.Sdk": "6.0.0-beta.21405.3",
-    "Microsoft.DotNet.PackageValidation": "1.0.0-preview.7.21352.4",
     "Microsoft.DotNet.Arcade.Sdk": "6.0.0-beta.21405.3",
     "Microsoft.DotNet.Helix.Sdk": "6.0.0-beta.21405.3",
     "Microsoft.DotNet.SharedFramework.Sdk": "6.0.0-beta.21405.3",