Clean up github repo

[fkobi]

I think the github repo could be improved by removing the unused units:

Issues

The readme points to sourceforge's bug tracker which seems to be active, contrary to github's.

freecol/README.md

Line 54 in e59c472

* Report any bugs you find to our [Bug Tracker.](https://sourceforge.net/p/freecol/bugs/)

I think that github tab should be removed.

Also the issues shoud be garbage-collected IMO.

Projects

It is unused, can be removed.

Security

With how barebones it is it might be worth it to just put the email in the usual readme.
Also 1.2.0 should be probably added there.