Removed minVertexIndex Exception

This removes the exception on minVertexIndex in GraphicsDevice.DrawIndexedPrimitives().

minVertexIndex and numVertices are simply hints for software vertex processing.

 http://blogs.msdn.com/b/jsteed/archive/2004/07/16/drawindexedprimitive-demystified.aspx

> The MinIndex and NumVertices values are really just hints 
> to help Direct3D optimize memory access during software 
> vertex processing, and could simply be set to include the 
> entire vertex buffer at the price of performance.

Since DirectX11 and OpenGL do not support such hints these parameters are currently unused in MonoGame.  We leave them only for compatibility with XNA.