RFR: 8212701: remove sun.desktop property from launcher code

[Phil Race]

Bug: https://bugs.openjdk.java.net/browse/JDK-8212701
CSR: https://bugs.openjdk.java.net/browse/JDK-8222814
Webrev: http://cr.openjdk.java.net/~prr/8212701/index.html

Please review the above CSR and webrev which eliminates the sun.desktop 
system property
which was set in launcher code to the value
1) "windows" on Windows for no apparent reason
2) "gnome" if running in a Gnome desktop session
3) left unset in all other cases/platforms/desktops, so it being null 
seems unlikely to be a problem.

The Swing code in the desktop module that consumed the value
has been updated to get it in a different way internal to the desktop 
module.

-phil.