cogl: Remove unused cogl_framebuffer_set_dither_enabled

GL_DITHER is enabled by default so this is no change.

GNOME/mutter!813
5 jobs for !813 with cogl-framebuffer-state-cleanup in 7 minutes and 37 seconds
detached
Status Job ID Name Coverage
  Review
passed #565068
check-commit-log

00:00:32

 
  Build
passed #565069
build-mutter

00:03:28

passed #565070
build-without-native-backend-and-wayland

00:03:09

 
  Test
passed #565072
can-build-gnome-shell

00:01:36

passed #565071
test-mutter

00:03:36