Remove the unused AvaloniaEdit.TextMate.Grammars package reference #545

Found during the Avalonia 12 upgrade planning. PlanViewer.App.csproj references AvaloniaEdit.TextMate.Grammars 0.10.12.1, but no app code uses it: the package ships its own RegistryOptions/ThemeName under the AvaloniaEdit.TextMate.Grammars namespace, while every call site binds TextMateSharp.Grammars instead (QuerySessionControl.Editor.cs uses it via a using directive; PlanViewerControl.Schema.cs fully qualifies it). The two never collide because the AvaloniaEdit namespace is never imported. Removing the reference also drops Newtonsoft.Json 13.0.1 and TextMateSharp 1.0.23 from the resolved graph, which is why it was kept out of the migration PR (a runtime-graph change in a release meant to be nothing but the migration). Safe cleanup for a normal release.