Remove gtk# dependency. Now (mostly) handled by SixLabors.ImageSharp

Author: decriptor

src/TaglibSharp.Tests/MainWindow.cs

@@ -3,7 +3,6 @@
   <PropertyGroup>
     <TargetFrameworks>net461;netcoreapp3.1</TargetFrameworks>
     <IsPackable>false</IsPackable>
-    <AssemblySearchPaths>$(AssemblySearchPaths);{GAC}</AssemblySearchPaths>
   </PropertyGroup>
 
   <ItemGroup>
@@ -15,7 +14,6 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="GtkSharp" Version="3.22.25.74" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.6.1" />
     <PackageReference Include="NUnit" Version="3.12.0" />
     <PackageReference Include="NUnit.ConsoleRunner" Version="3.11.1" />