It's been hinted at, but it hasn't been fully touched on by the other answers: it all boils down to cost, and some of the costs are trickier than you might think.
The most obvious cost is that someone has to delete them. Now you could just put an intern on the case and have him go searching through files and whatnot to remove the ones that aren't needed in the actual game. But still, you have to pay for this and, even if you don't (because you're a monster who doesn't pay interns) you still could have had that intern doing something useful like ordering pizza for the dev team, so it still costs you.
The less obvious cost is that the resource you're about to remove was put in for a reason: are you absolutely sure that reason is obsolete? Sure, you wrote this code for a demo and don't need it anymore, but is that entirely true? Did you write a useful function that you forgot to migrate to the other libraries? Did Stacey make a really cool graphic for the demo that ended up in the real game? Does an internal simulator still make use of that old level you used for testing?
Making games is a business. Even if it isn't a money-making business, it's still a business with limited resources. Every decision requires a cost-benefit analysis, even if you don't go writing business cases for each one. And the simple fact is that cost of leaving those assets in the game is a slightly larger download, and the benefits are a significantly reduced risk of something blowing up right before release.
All those question marks in the "less obvious" paragraph represent uncertainty. You just don't know. And when you're running a business, you have to trade your unknowns for knowns. And what is known is that the package you built passed testing, even if it has a little bloat to it.
There other potential costs such as information leaks around the development process that could potentially be used to derive hacks, but you made it through the testing process as-is, you shouldn't really be worried about that.
tl;dr: No one will notice a 4% larger download, but they will notice if you broke the game by removing necessary files.