[FNA] Remove unused defs for Storage

Kills WINDOWS_STOREAPP, PSM, and WINRT paths not relevant to FNA.  Removed
code was specific to mobile stores and not relevant to open FNA targets.
Eliminates SDL2 def checks, as it is now the standard path.
[FNA] Remove unnecessary def's.

Only one change - eliminate DIRECTX path in VertexElement.
Merge pull request #153 from 3vi1/monogame-sdl2

[FNA] Remove unused defs - Storage & Graphics/Vertices