drm/info: Remove unused code

Commit 28a62277 ("drm: Convert proc files to seq_file and introduce
debugfs") converted /proc files to debugfs and in the process dropped
the entry for the vblank statistics. Since that file has been gone for
almost five years, there is no use to keep the code that provides the
file's content around.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>