[scenes/opengl] Remove glDrawBuffer call

Summary:
According to Gl 3.2 (page 501) and 4.5 (page 204) specs the initial state of
the default framebuffer is already BACK. Therefore we do not need to set it
explicitly.

When we draw in the future to alternative framebuffers which do not have back
buffers this call is fatal.

Test Plan: No tearing on Wayland, tearing as before on X11.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D25868