EasterEgg: Call onDelete if removed

[robothauler]

Fixes #4329

[impaktor]

Ah, good. I had a feeling I've missed something. In fact, I was thinking, it shouldn't need saving/loading state at all, since it's fixed to the station, and never goes away, or changes, so I had a thought some time ago, to remove the seralize/unserialize functions.

[robothauler]

I pushed it again because I forgot onGameStart.

[fluffyfreak]

Code looks good to me, but I'll leave this review to @impaktor :)

[impaktor]

👍 for branch name!

[impaktor]

So, the bug was, if visiting a system with an "egg", then visiting another system, the BBS would try to use that egg, I think, (just running the code very sloppily in my brain).