xen/grant-table: remove explicit linear mapping additions

There's no need to explicitly add linear mappings for the grant table
area, as the memory is allocated using xenmem_alloc and it should
already have a linear mapping that can be obtained using
rman_get_virtual.

While there also remove the return value of gnttab_map, since there's
no return value anymore.

Sponsored by: Citrix Systems R&D
Reviewed by: Elliott Mitchell <ehem+freebsd@m5p.com>
Differential revision: https://reviews.freebsd.org/D29602