Revert "[deps] roll dartoc to a more recent version"

This reverts commit 2759a44.

Reason for revert: I see many luci failures w/ this; not sure its related, but reverting for now.

Original change's description:
> [deps] roll dartoc to a more recent version
>
> Change-Id: I31e9bc532a9d67bfb2424cb66b18861d09da301f
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/448422
> Auto-Submit: Devon Carew <[email protected]>
> Reviewed-by: Konstantin Shcheglov <[email protected]>
> Commit-Queue: Konstantin Shcheglov <[email protected]>

No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: Ibd39a8ad381c92f81f633fb3dd2e6f3187ecad29
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/448424
Auto-Submit: Devon Carew <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>

Author: devoncarew

DEPS

@@ -133,7 +133,7 @@ vars = {
   # EOL comment after a dependency to instead pin at the current revision.
   "ai_rev": "1547a834d8a3bb021f2e6c49c41d5aaad11a5b7f",
   "core_rev": "5c3e2c38df268be2347f3aad30ced0147dd012bb",
-  "dartdoc_rev": "8b4d03e694beb5b9ece69d0dd9100a84bd10cd57", # https://github.com/dart-lang/dartdoc/issues/4100
+  "dartdoc_rev": "d683b47abc407a3093c269f67ed4c66b98138d56",
   "ecosystem_rev": "36e514d52c5d056227e4cea98b784ade50b5b4f1",
   "flute_rev": "d5adc4d6439572db5da71a9261b9bf0c7c96daa4",
   "http_rev": "ef05b3744424885d93f88a6a50664fb5b7d5cbdb",