Removing AfterBuild CustomCommands

Removed the parts below (twice) as they break the build on Xamarin Studio (and removing them doesn't break the build on VS)
Also, as far as I understand, a non-Windows OS won't have that conflict and won't need using this assembly in the first place.
```
<CustomCommands Condition="'$(OS)' != 'Windows_NT'">
  <CustomCommands>
    <Command type="AfterBuild" command="mv System.Net.Droid.dll System.Net.dll" workingdir="${TargetDir}" />
  </CustomCommands>
</CustomCommands>
```