cogl/driver: Remove GError from context_init

It is not used by anyone, let's just remove it.

!1152
6 jobs for !1152 with gbsneto/sorry-about-that in 20 minutes and 44 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Review
passed #643918
check-commit-log

00:00:37

 
  Build
passed #643919
build-mutter

00:16:33

passed #643921
build-without-native-backend-and-wayland

00:07:25

passed #643920
build-without-opengl-and-glx

00:15:32

 
  Test
passed #643923
can-build-gnome-shell

00:02:06

passed #643922
test-mutter

00:03:33