D3D12: Remove feature level checks

We don't create a device below feature level 11_0 anyway, so no point
checking, we can just assume support.

Source/Core/VideoBackends/D3D12/D3DBase.h

@@ -66,7 +66,6 @@ void UnloadDXGI();
 void UnloadD3D();
 void UnloadD3DCompiler();
 
-D3D_FEATURE_LEVEL GetFeatureLevel(IDXGIAdapter* adapter);
 std::vector<DXGI_SAMPLE_DESC> EnumAAModes(IDXGIAdapter* adapter);
 
 bool AlertUserIfSelectedAdapterDoesNotSupportD3D12();