Do not disable egl on desktop gl automatically

desktop opengl and egl are not incompatible with eachother, so there
is no need to disable egl when on desktop opengl

Task-number: QTBUG-34949

Change-Id: I757c38674a480910d1cb23853c255eb993e107ff
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Andrew Knight <andrew.knight@digia.com>
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>