xserver_xorg-server: Remove unused null root cursor option

Upstream removed this configure option back in 2010 (for server 1.8):

http://cgit.freedesktop.org/xorg/xserver/commit/?id=60b6477f6dc005a1b052be8c5



So this hasn't done anything since 759d548e (xserver_xorg-server: bump
version).

Don't add any Config.in.legacy for this as it as been broken for so long any
way.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>