[pkg] remove vendoring of getlantern/systray

after
https://github.com/getlantern/systray/commit/5fb0feca3c0677e9fa31e579ff69631f49a379a2
was merged, we don't need to keep using the vendoring of systray.

there is some flickering with the stop/start/cancel menu items
chening their orderning that still needs to be solved.