<AWT Dev> RFR: 8154546: Retire sun.misc.GThreadHelper

[Phil Race]

Bug: http://cr.openjdk.java.net/~prr/8154546/
Webrev: http://cr.openjdk.java.net/~prr/8154546/

The options were move this or delete it.
There is some discussion in the bug report about GLIB versions
but the only client of it was FX and that no longer uses the class
as it supports only newer Linuxes. So I am deleting the class.
There was a CCC about adding it for FX so I will file a follow-on CCC to 
say its gone ..

-phil.