drm: Drop drm_sysfs_class from drmP.h

No user at all.

My guess is that this is a leftover from ttm before it used
the more abstract helpers to register/unregister its sysfs
objects (see drm_sysfs.h). At least in the existing history
it was never used.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>