net-libs/tox should use .service provided by toxcore rather than own

[zetok]

1. There's a problem with service "bundled" in repo - it uses /var/run for storing data, which by some conventions is a symlink to /run, which, under systemd is a tmpfs. May not be a real problem for Gentoo for some time, but still, it would be better to use something different.
2. There's absolutely no need to keep .service in this repo - toxcore repo provides one, along with config, etc. What may be needed, is actually fixing .service provided by toxcore, since I've heard that it doesn't work correctly.

• use .service provided by toxcore
• use config file provided by toxcore (small patch adjusting it may or may not be needed to be included here)

Those changes should be made, unless there is a reason to not do that?