When it comes to the frontend code you always minify it (remove white spaces, comments etc) in production.
Should one do the same with server code? I usually have a lot of comments in my server files. But I have never heard about people doing so.
Wouldn't the server run faster if the code was optimized in the same way?
You're not going to have any improvement as the whitespaces and all formatting are lost when your server side code is translated to machine code (or interpreted). It's also not sent over the wire, it's read from the local filesystem, so while having less characters would lead to a faster startup, it would not make any difference on the long run and the startup speed gain would be marginal (or even unnoticeable).
So, no, minifying your server side code is basically useless, worse, it's probably going to make stack traces completely useless, as there's going to be a lot of code in the same line (and not necessarily with the same formatting you used).