README.md: remove icons

[kevinburke]

I don't think they add much in particular and they make us look bad if
the build is failing. It's easy to see whether the build is failing
because the build status is present on any PR/tip commit on master,
and you can tell we use prettier because the build will fail if your
formatting doesn't match it.

This PR does not need to update the CHANGELOG because it's trivial.

[felixfbecker]

I don't think they add much in particular and they make us look bad if
the build is failing.

Yeah, and they should make us look bad if the build is failing. If the build is failing and that makes us look bad the solution is to fix the build, not remove the badge. That's like putting tape over the lamp in your car telling you your engine needs maintenance. A build badge is a useful indicator of whether I can currently build the project from master, and they are so ubiquitous that not having one makes us look bad. If I don't see a build badge in a project I literally assume it has no CI (and I would not check the commit page first). Also, we use badges in every other of our repos, even though the same argument could be made everywhere. In fact, you could make that argument for every open source project out there, yet every open source project I can think of has them, so clearly the majority of people like them. I was okay with removing the codecov badge which revealed an ugly truth about our codebase, because it is not easily fixable. But the build badge should show a green status 99.99% of the time, so its net impact on people's impression of our repo will be positive, not negative.