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.

gcampax/cogl,collects/cogl,gcampax/cogl,djdeath/cogl,spatulasnout/cogl,djdeath/cogl-android,djdeath/cogl,collects/cogl,Distrotech/cogl,Distrotech/cogl,djdeath/cogl-android,spatulasnout/cogl,djdeath/cogl,gcampax/cogl,djdeath/cogl-android,gcampax/cogl,Distrotech/cogl,Distrotech/cogl