[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.
removing #ifs for Input\Touch and Media
[FNA] Remove unnecessary def's.

Only one change - eliminate DIRECTX path in VertexElement.
Merge pull request #154 from extrahotchilipowder/monogame-sdl2

removing #ifs for Input\Touch and Media
Merge pull request #153 from 3vi1/monogame-sdl2

[FNA] Remove unused defs - Storage & Graphics/Vertices
Merge pull request #155 from khbecker/monogame-sdl2

[FNA] License Cleanup for Input and Input/Touch.