Remove useless instantiation of a GC

create a new GC and disposing it can be costly depending on the platform
(as a matter of fact it is quite costly on linux)
After a profiling session of the theme while resizing the IDE window I
noticed that

net.jeeeyul.eclipse.themes.rendering.HackedCTabRendering.computeTrim(int,
int, int, int, int, int)

is creating a new GC and disposing it right away never using it.