Remove unused code from Texture2D.DirectX and do a minor tidy up

Fixes #2306

Removed the MONOMAC regions.
Removed the extra way to load in PlatformFromStream.
Did a minor tidy up on CreateTex2DFromBitmap / LoadBitmap (including whitespace).
Tidy up usings.

View without whitespace changes:
https://github.com/danzel/MonoGame/commit/fc379d30edd72b274c31dcf7c29a64553f4dd560?w=1