agp: kill agp_(map|unmap)_page

Only used to remap the scratch page. Now that intel-gtt does this
itself, kill the support code.

Cc: Dave Airlie <airlied@linux.ie>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>