net-im/telepathy-mission-control: Drop USE=upower

net-im/telepathy-mission-control: Drop USE=upower

If built with --disable-upower, telepathy-mission-control will listen for
PrepareForSleep signal instead, which is now also available through
ConsoleKit2. Also get rid of USE=upower since it only meant legacy-upower
and USE=systemd now since it doesn't serve a purpose with upower removed.

References:
https://bugs.freedesktop.org/show_bug.cgi?id=70458
http://consolekit2.github.io/ConsoleKit2/#Manager::PrepareForSleep

Package-Manager: portage-2.2.24