Remove unused AstcSharp reference

Author: Erik-White

src/ImageSharp.Textures/ImageSharp.Textures.csproj

@@ -39,7 +39,6 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="AstcSharp" Version="0.9.7" />
     <PackageReference Include="SixLabors.ImageSharp" Version="3.1.12" />
   </ItemGroup>