deskutils/vboxgtk: remove bogus dependency

pytgk2 is not used by this port, so remove it.
The port is marked broken already (fetch fails), but it also broken

in another way:

> vboxgtk-0.8.2_1 depends on file: /usr/local/lib/python2.7/site-packages/xpcom/vboxxpcom.py - not found

• Error code 1