[WP8] Set the correct GraphicsProfile and FeatureLevel
Remove member _featureLevel.

Remove member _featureLevel.

The only places it's used is when we update it and right after that it's
used to set the GraphicsProfile. There is no reason to keep a  local
copy. It can always be accessed via _d3dDevice.FeatureLevel.
Merge pull request #2292 from nkast/tnc_WP_SetFeatureLevelAndGraphicsProfile-

[WP8] Set the correct GraphicsProfile and FeatureLevel
Merge branch 'develop' of https://github.com/mono/MonoGame into toRelease