amd64/efi: Remove setting hints for rsdp

Given that hints set this way don't work when a static kenv is compiled
into the kernel. acpi.rsdp has been set for this for the past 6 years,
and all kernels in that time have used it in preference to the hints. As
such, we no longer hints.*, so remove them.

Sponsored by: Netflix
Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D35634