[css2][css-display-3] Remove "display:run-in" from CSS

tl;dr: could we remove display:run-in from CSS specs, to reflect reality?

See https://bugzilla.mozilla.org/show_bug.cgi?id=2056#c97 for some background. As Joe Walker noted there (emphasis added):

Partial support for display:run-in was removed from WebKit and Blink many years ago. IE11 (pre-Blink) had (some) support, however as IE11 is not actively maintained, there are zero modern browser engines with support for this feature and we are unaware of any interest in reviving it as specified.

We believe that display:run-in should be removed from any CSS specs being maintained. If there is web developer/author interest in this sort of "run-in" styling functionality we should redesign a new feature that is based on the host language having a definition/concept of "paragraph" as noted for example in https://html.spec.whatwg.org/multipage/dom.html#paragraph.

CC @fantasai @tabatkins @tantek @emilio

Start by locating every display: run-in definition and reference in the maintained CSS specs, then read the issue and its linked Bugzilla discussion for scope. Done means the obsolete feature is removed from the relevant specs and any affected references are updated consistently.