To reproduce:
1. In Project window, create a new material
2. In material's Inspector window, check Emission parameter
3. Expand newly appeared Global Illumination parameter's options

Expected: three available options: None, Realtime, Baked
Actual: two available options: Realtime and Baked. None option missing (check picture attached)

Reproduced in: 5.6.0b2, 5.6.1f1, 5.6.2p2, 2017.1.0f3, 2017.2.0b2
Not reproduced in: 5.4.5p4, 5.5.4p1, 5.6.0b1
Regression starts in: 5.6.0b2

Note: If the feature was meant to be removed, documentation needs to be updated as it still states 3 options should be available
Note: related documentation link: https://docs.unity3d.com/2017.1/Documentation/Manual/StandardShaderMaterialParameterEmission.html