Doesn't matter if the TargetFrameworkVersion is still there.
I forgot one thing, assuming you're modifying an existing .csproj file that was not created from one of the Mono projects templates in SharpDevelop, you'll need to replace
<Import Project="$(MSBuildBinPath)\Microsoft.CSharp.Targets" />
with
<Import Project="$(MonoBuildTasksPath)\Mono.Build.CSharp.targets" />
If you're creating a new project from scratch it is easier to use the existing Mono project template. By default they target Mono 2.0.