Remove net_2_0 and net_3_5 dependencies in mono build; mono eliminated them

master mono has removed net_2_0 and net_3_5 profiles, so it is unable to build fsharp with master mono anymore because fsharp on mono seems to make use of net_2_0 to bootstrap build. So, remove them.
This is now duplicating Line 29. But why do we need to remove it?  The condition should just return false if Mono .NET 2.0 is not available.