drop OnlyShowIn from .desktop files

<mclasen> I see libreoffice installs desktop actions with OnlyShowIn=Unity
<mclasen> I believe OnlyShowIn for actions was kicked out when they got merged in the spec
<caolan>  mclasen: so drop the "OnlyShowIn" lines ?
<mclasen> would be my recommendation

Change-Id: I5d119d361be197bac3fc20c08b4e43f11f4b0c0c