cogl: Remove reference to nonexistent type CoglVertex2f

It is actually CoglVertexP2.

GNOME/mutter!979
5 jobs for !979 with no-CoglVertex2f in 7 minutes and 5 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Review
passed #533161
check-commit-log

00:00:24

 
  Build
passed #533162
build-mutter

00:03:33

passed #533163
build-without-native-backend-and-wayland

00:01:19

 
  Test
passed #533165
can-build-gnome-shell

00:01:23

passed #533164
test-mutter

00:03:07