Remove <hr> declarations

[atjn]

I believe the <hr> declarations do more harm than good. I have made a test document to show what I mean.
Source: https://codepen.io/atjn/pen/qBzXydJ

In these pictures, you see a comparison between Firefox (left), WebKit (middle), and Chromium (right).

In the first picture, the modern-normalize declarations are not enabled:

As you can see, all browsers display the <hr> in the same way, so there is no need for normalization. (There is one exception when applying a color, but that can't be fixed with CSS, it requires changes to Chromium)

In the second picture, the modern-normalize declarations are enabled:

First, you can observe that the height: 0px declaration is no longer needed, since modern Firefox already uses that.
Second, you will notice that the color: inherit declaration did not normalize anything, instead it created a new compat issue when colors are inherited.

It might make sense to add color: inherit in #13 (modern-base) because it provides a better default, but it does not belong in the normalization sheet.