remove IntelliJ project folder and add let git ignore it in the future

[nitram509]

Since this is a Maven-based build system, the IntelliJ project files and folder should not be committed to git. Also, while looking into the ed25519.iml file, I realized different dependencies are used. That might lead to confusion for other developers.

Any feedback welcome

PS: When applying this patch, this will break the project files for the ones, using Intellij. But, thatäs not a big deal, since it just takes a handful of clicks to re-import it.