Drop legacy fixup rule for VirtualBox.

Summary:
Versions of VirtualBox packaged on Neon and Fedora now have
a proper WM_CLASS:
Fedora: WM_CLASS(STRING) = "VirtualBox", "VirtualBox"
Neon: WM_CLASS(STRING) = "Qt-subapplication", "VirtualBox"

And install a matching virtualbox.desktop.

We can therefore drop this rule.

BUG:350468

Side note: The rule should actually still have worked, but
for some reason KServiceTypeTrader refuses to match a .desktop
with 'Exec=VirtualBox %U' when we query for 'VirtualBox', as
extracted from cmdline. I'm talking to dfaure about it.

Reviewers: Plasma, broulik

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D4346