Reland "[Fuchsia] Remove the legacy gn build rules for fuchsia"

This change may require a codereview from flutter side before moving
forward. See https://github.com/flutter/engine/pull/51072.

This reverts commit 283051acba091ca172d089cecf6bbe401030014e.

Reason for revert: Redo the change with the similar one in flutter.

After this change, the next dart -> flutter roll needs extra cares
in https://github.com/flutter/engine/pull/51072. It also contains
more details about the reason why it could not be two-way
compatible.

Original change's description:
> Revert "[Fuchsia] Remove the legacy gn build rules for fuchsia"
>
> This reverts commit 094202bb91f881e7a4a3b44246842dfed9370fce.
>
> Reason for revert: Break flutter, this change should happen after
> flutter being migrated to the same gn-sdk.
>
> Original change's description:
> > [Fuchsia] Remove the legacy gn build rules for fuchsia
> >
> > The updated test-scripts contains https://crrev.com/c/5341620 which
> > allows to generate fidl apis with an env var controlled location
> > rather than a hard-coded one. So the existing fuchsia gn build rules
> > in dart/sdk become obsolete and can be fully removed in favor of the
> > one in //third_party/fuchsia/gn-sdk/.
> >
> > Meanwhile the gn-sdk has been updated with
> > https://crrev.com/c/5325282 to use api-level version'ed idk but not
> > the ones in arch/.
> >
> > Cq-Include-Trybots: luci.dart.try:vm-fuchsia-release-arm64-try,vm-fuchsia-release-x64-try
> > Tested: ^^^^^
> > Bug: 40935282
> > Change-Id: I2ce958e6db1ff8221beef7b7ff953c32bb4525ba
> > Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/355283
> > Reviewed-by: Ryan Macnak <rmacnak@google.com>
> > Reviewed-by: Alexander Thomas <athom@google.com>
> > Commit-Queue: Zijie He <zijiehe@google.com>
>
> Bug: 40935282
> Change-Id: I7c455d1d362210523671c97d99ef018ede1743f4
> Cq-Include-Trybots: luci.dart.try:vm-fuchsia-release-arm64-try,vm-fuchsia-release-x64-try
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/356307
> Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
> Auto-Submit: Zijie He <zijiehe@google.com>
> Reviewed-by: Derek Xu <derekx@google.com>
> Commit-Queue: Zijie He <zijiehe@google.com>

Bug: 40935282
Change-Id: Id5a9a98013350359037b19f8506f24158c9a3120
Cq-Include-Trybots: luci.dart.try:vm-fuchsia-release-arm64-try,vm-fuchsia-release-x64-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/356924
Commit-Queue: Zijie He <zijiehe@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>