Remove deprecated platforms

The "Windows Phone" and "Windows Phone Silverlight" platforms are no longer
relevant. After talking to the Windows team it seems it's easier to remove
these two rows from the support table.

Author: Immo Landwerth

docs/versions.md

@@ -26,8 +26,6 @@ most target .NET Standard 1.1.
 |Xamarin.Android                       |    7.0  |    7.0  |  7.0    |    7.0  |    7.0  |   7.0        | **7.0**      | **8.0**      |              |
 |Unity                                 | 2018.1  |  2018.1 |  2018.1 |  2018.1 |  2018.1 |       2018.1 | 2018.1       |**2018.1**    |              |
 |Universal Windows Platform            |    8.0  |  **8.0**|**8.1**  |   10.0  | **10.0**|  10.0.16299  |  10.0.16299  |**10.0.16299**|              |
-|Windows Phone                         |    8.1  |    8.1  |**8.1**  |         |         |              |              |              |              |
-|Windows Phone Silverlight             |  **8.0**|         |         |         |         |              |              |              |              |
 
 [1.0]: versions/netstandard1.0.md
 [1.1]: versions/netstandard1.1.md