Drop MOUSE_GETVARS and MOUSE_SETVARS ioctls support.

These ioctls are not documented and only stubbed in a few drivers: mse(4),
psm(4) and syscon's sysmouse(4). The only exception is MOUSE_GETVARS
implemented in psm(4)

Given the fact that they were introduced 20 years ago and implementation
has never been completed, remove any related code.

PR: 228718 (exp-run)
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D15726