char/agp: Remove frontend code

The AGP subsystem supports a user-space interface via /dev/agpgart. It
is only enabled with DRM support for mode setting in user space. (i.e.,
CONFIG_DRM_LEGACY). All of that DRM code has been removed and the option
will go away. Hence remove the AGP frontend.

Modern DRM drivers with kernel mode setting handle AGP support internally.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: David Airlie <airlied@gmail.com>
Reviewed-by: Daniel Vetter <daniel@ffwll.ch>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231122122449.11588-14-tzimmermann@suse.de