fix build warning

To fix
Warning	3 "C:\windows\Microsoft.NET\Framework\v4.0.30319\Microsoft.CSharp.targets" cannot be imported again. It was already imported at "[...]gitextensions\GitExtensionsTest\GitCommands\GitCommandsTest.csproj (43,3)". This is most likely a build authoring error. This subsequent import will be ignored.
I removed a line from the csproj file