emulators/virtualbox-ose: Remove unnecessary dependency on 32-bit compat for amd64

32-bit in-kernel support and lib32 libs are not necessary for VirtualBox 6.x,
so just remove this requirement.
Also the port support amd64 only - remove I386 from PLIST_SUB.

PR: 266382
Tested by: Olivier Certner <olivier.freebsd@free.fr>
Approved by: arrowd (mentor)
Differential Revision: https://reviews.freebsd.org/D40487