[css-scoping-1] Drop :host-context

[rniwa]

Splitting the discussion from #1915, as we've repeatedly said in the past, we don't think changing the style of an element based on where it appears is a good practice to design a re-usable element in general, and this particular feature poses a significant implementation complexity & cost as pointed out by @emilio in #1915 (comment).

We propose to drop this feature from the specification as we've previously requested.

[rniwa]

@hober @smfr @litherum

[emilio]

Dupe of #1914?

[tabatkins]

Yeah, dupe.

Note @emilio's comment in #1914; we'll put in usage counters to see if it's easily droppable or if it should be reviewed more closely. I filed https://bugs.chromium.org/p/chromium/issues/detail?id=936225 to track it.