Revert "Give <slot> element default unicode-bidi: isolate style."

This reverts commit 198d41cfd6eac5fd9ee34e6314e34f938b6d31b5.

Reason for revert: This reverts a change that was the first of two closely related changes.  Given that the other change, in https://crrev.com/c/4973701, turned out to have problems (and I haven't attempted to land it), I've proposed reverting both in the HTML spec at whatwg/html#9880.  I'm therefore reverting this (the first half) until the HTML spec discussion is sorted out.

Original change's description:
> Give <slot> element default unicode-bidi: isolate style.
>
> This is behind the RuntimeEnabledFeatures::CSSPseudoDirEnabled() flag.
>
> This implements a part of the behavior described in:
> whatwg/html#3699 (comment)
> which has been specified in:
> whatwg/html#9554
> whatwg/html#9796
>
> Bug: 576815
> Change-Id: Ice471838c7251fc69d87abfa723f3eef1a4c455a
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4800075
> Reviewed-by: Chris Harrelson <[email protected]>
> Reviewed-by: Di Zhang <[email protected]>
> Commit-Queue: David Baron <[email protected]>
> Cr-Commit-Position: refs/heads/main@{#1214439}

Bug: 576815
Change-Id: I0f52b30c345d4962a36a36e5d2651221db96bcde
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4974023
Reviewed-by: Philip Rogers <[email protected]>
Bot-Commit: Rubber Stamper <[email protected]>
Auto-Submit: David Baron <[email protected]>
Commit-Queue: Philip Rogers <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1215009}