Tidying up using statements

As mentioned by @flibitijibibo in #2218, we have some unnecessary using statements left over from before the file splits.
I've gone through and tidied them up (I may have gone a bit overboard in finding them).

As I was looking at all the split files, I also did some very minor code tidy ups:
OpenTKGamePlatform.cs - Moved Linux OnIsMouseVisibleChanged beside the windows one.
RenderTarget2D.DirectX.cs - Removed an early return that wasn't doing anything.
Texture.OpenGL.cs - Removed setting variable to default, using this when not needed.
VertexDeclaration.OpenGL.cs - Use a using to rename a type to make the code the same in OpenGL/GLES