fixed build errors relating to previously removed DataStructures project

CSharpCompat.fs: removed 'open FSharpx.DataStructures' as this namespace
nolonger exists.

FSharpx.sln: deleted Samples\DataStrutures project as DataStructures
proj no longer exists.

FSharpx.CSharpTests\FSharpx.CSharpTests.csproj and
CSharpTests.RoseTreeTest.cs: removed reference to RoseTreeTest.cs as
RoseTree.cs no longer exists