atkbd: remove usage of x86bios

Instead of trying to get the keyboard repeat rate set by the BIOS just set a
default one. This allows removing the usage of x86bios from atkbd.

Sponsored by: Citrix Systems R&D
Reviewed by: jkim, delphij
Differential Revision: https://reviews.freebsd.org/D2399
MFC after: 2 weeks