Remove IsPackable from test project

IsPackable automatically defaults to false for test projects, see https://github.com/NuGet/NuGet.Client/blob/6.13.0.69/src/NuGet.Core/NuGet.Build.Tasks.Pack/NuGet.Build.Tasks.Pack.targets#L35C6-L35C16

Author: 0xced

tests/Serilog.Formatting.Log4Net.Tests.csproj

@@ -3,7 +3,6 @@
   <PropertyGroup>
     <TargetFramework>net8.0</TargetFramework>
     <Nullable>enable</Nullable>
-    <IsPackable>false</IsPackable>
   </PropertyGroup>
 
   <ItemGroup>