Remove bad target framework declaration

Author: JimBobSquarePants

tests/ImageSharp.Drawing.Tests/ImageSharp.Drawing.Tests.csproj

@@ -13,7 +13,6 @@
   <Choose>
     <When Condition="$(SIXLABORS_TESTING_PREVIEW) == true">
       <PropertyGroup>
-        <TargetFrameworks>net8.0</TargetFrameworks>
         <TargetFrameworks>net8.0;net9.0</TargetFrameworks>
       </PropertyGroup>
     </When>