fb: Remove unused cursors ioctls

The cursors related ioctls were added in 1994 with the 4.4 Lite import
but were never implemented in either sc(4), fb(4) or vt(4).
Remove it.

Reviewed by: emaste, tsoome
MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D33217