If, besides the few bytes of HD space, it has 0 impact on the performance then there probably is no real point in cleaning them up. (as long as they aren't simply wrong)
Look, you're following what I just said in topic #9262 too .... much. At least, the reasoning you're giving (server load/space) is too .... wrong, but in a newbish everyone-starts-out-thinking-that-way way. The alias/implication links do matter for the server, but that's for albert, jxh and others who help with dev work to worry about; it's not something you factor into when deciding on any one link.
This is really a computer science thing and it was ages since I've had to explain this stuff, but it's algorithms and process that made software efficient; focusing on tiny little imperfections in code and databases is probably wasting your time when there are other things that need your attention. Either it's completely central to the task at hand, or it really really just don't matter at all.
Kind've more on topic, those are plural-singular aliases and they generally serve a purpose. It's whether the links keep Danbooru more organised that matters, and these links kinda do.
Edit: Woah what? Why does it say I edited the OP? I never/can't touch it.
Well, yes, I've never worked with databases before, so i asked :)
I was thinking something along the lines of "every time a post is edited, it has to check all aliases -> removing like 50/8000 aliases could speed up things a tiiiiny bit". (and it conflicts with my ocd :P)
Oh right, then yeah the specific examples you gave are useful aliases and even though they're not doing much right now, they're future proofing. Maybe. Just think of girls_und_panzer and all previously obscure tank-specific tags that sprang up in response to that.