gsdx ogl: remove useless ps.rt

Note: I think we can do the same on DX11

Perf wise: on colin mcrae 3 it reduces shader prog setup from 3005 to 2086 each frames. It saves 2 ms of CPU processing (27->29fps)



git-svn-id: http://pcsx2.googlecode.com/svn/trunk@5714 96395faa-99c1-11dd-bbfe-3dabce05a288