Revert "Add html_common to .gni files."

This reverts commit 75e1f2a.

Reason for revert: Fix for dartdoc #1949 makes this no longer strictly necessary.  While this leaves the underlying imports unfixed, I will instead file an issue against the engine.

Original change's description:
> Add html_common to .gni files.
>
> This will enable a fix to an unresolved export that causes Dartdoc
> to complain about all Flutter packages.
>
> Bug: dart-lang/dartdoc#2143
> Change-Id: I634e21066420b100295989cf9bdcd35af91a617b
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/143020
> Reviewed-by: Devon Carew <[email protected]>
> Commit-Queue: Janice Collins <[email protected]>

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: dart-lang/dartdoc#2143
Change-Id: Ibe25eee8031b348d9260245daca84d9742fbb894
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/143402
Reviewed-by: Mike Fairhurst <[email protected]>
Commit-Queue: Janice Collins <[email protected]>

Author: jcollins-g