Remove _cogl_flush_matrix_stacks from cogl-internal.h
Remove _cogl_flush_matrix_stacks from cogl-internal.h

The definition for this function was removed in f7d64e5abvoid so it
should be removed from the header too.

Distrotech/clutter,jigpu/clutter,spatulasnout/clutter,GNOME/clutter,collects/clutter,jigpu/clutter,rkudiyarov/clutter_osx,spatulasnout/clutter,djdeath/clutter-android,djdeath/clutter-android,djdeath/clutter,collects/clutter,GNOME/clutter,jigpu/clutter,nobled/clutter,dlespiau/clutter,djdeath/clutter-multithreaded,GNOME/clutter,jigpu/clutter,heysion/clutter-clone,jigpu/clutter,collects/clutter,collects/clutter,GNOME/clutter,Distrotech/clutter,spatulasnout/clutter,Distrotech/clutter,djdeath/clutter-android,djdeath/clutter,heysion/clutter-clone,rkudiyarov/clutter_osx,kerrickstaley/clutter-vala,ebassi/clutter,ebassi/clutter,dlespiau/clutter,Distrotech/clutter,rkudiyarov/clutter_osx,djdeath/clutter,kerrickstaley/clutter-vala,heysion/clutter-clone,kerrickstaley/clutter-vala,collects/clutter,djdeath/clutter,spatulasnout/clutter,spatulasnout/clutter,ebassi/clutter,dlespiau/clutter,GNOME/clutter,nobled/clutter,nobled/clutter,ebassi/clutter,djdeath/clutter-multithreaded,djdeath/clutter-multithreaded,djdeath/clutter-multithreaded,djdeath/clutter-android,djdeath/clutter,heysion/clutter-clone,djdeath/clutter-android,ebassi/clutter,jigpu/clutter,Distrotech/clutter,kerrickstaley/clutter-vala,rkudiyarov/clutter_osx,Distrotech/clutter,heysion/clutter-clone,djdeath/clutter