Deprecate KdTree3D worklets

The implementation of the search in the k-d tree is problematic because
it uses unbounded recursion. This is a problem for GPU devices, which
have very short stacks set by how many calls the compiler determines.
This is fixable, but the fix is not trivial.

This class is not used anywhere in VTK-m other than a trivial test.
Thus, I am just deprecating the class. I am also deleting the test, so
the code is not run anymore.