[poppler] GlobalParams: setters that are never called

[Albert Astals Cid]

In GlobalParams we have various setters that are never called, e.g.

GlobalParams::setScreenGamma

and then the getter is called from SplashOutputDev.cc

I suggest removing the GlobalParams api and for now adding a static const in 
SplashOutputDev.cc with the default value, we can add a setter to 
SplashOutputDev.cc if we find we need it sometime in the future.

Comments?

Albert