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