Retire support for Direct3D 9.x Feature Levels

[walbourn]

All shaders now build with the 4_0 profiles instead of the 4_0_level_9_1 profiles, and all classes that use shaders now require D3D_FEATURE_LEVEL_10_0 or greater. This reduces the footprint of the shader blobs by half.

Direct3D Hardware Feature Level 9.x was really last relevant for original Windows Phone and Surface RT, neither of which remain in the market. Some newer features of the toolkit (PBREffect, NormalMapEffect, and PostProcess already required 10.0+).

[walbourn]

walbourn/directxtktest#76