emulators/virtualbox-ose: remove extra dependency on cdrtools.

Dependency on cdrtools was introduced 6 years ago for sake of mkisofs:
https://svnweb.freebsd.org/ports/head/emulators/virtualbox-ose/Makefile?revision=274444&view=markup

In fact, VirtualBox build system does not use makeisofs
while building for FreeBSD because it does not even try to buid
VirtualBox Additions ISO image as its documentation says
in the Prerequisites section:

mkisofs (required only for additions which aren't supported yet)
=> /usr/ports/sysutils/cdrtools

https://www.virtualbox.org/wiki/FreeBSD%20build%20instructions

And it never really used mkisofs.
Drop the dependency and bump PORTREVISION.

PR: 225376
Approved by: vbox (maintainer timeout, 2 weeks)