xen/grant-table: remove explicit linear mapping additions

[royger]

Summary

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

Test Plan

Should be considered as a replacement for D29305

[ehem_freebsd_m5p.com]

I'm unaware of any problems with this. Further changes elsewhere are required to make this function with my aarch64 tree, but that is a separate commit and this appears fine to me.