cogl/renderer: Remove documentation of nonexistent enum

We don't want to delve into the philosohical study of the not-being,
so let's just not document an enum value that doesn't exist anymore.

!1151
6 jobs for !1151 with gbsneto/sharing-is-good in 9 minutes and 30 seconds (queued for 2 seconds)
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Status Job ID Name Coverage
  Review
passed #643638
check-commit-log

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  Build
passed #643639
build-mutter

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passed #643641
build-without-native-backend-and-wayland

00:01:41

passed #643640
build-without-opengl-and-glx

00:02:06

 
  Test
passed #643643
can-build-gnome-shell

00:02:06

passed #643642
test-mutter

00:03:27