Revert "[build] Debug means debug!"

This reverts commit 984c407.

Reason for revert:
This change causes failures on the flutter engine tests.
Reverting for now to unblock the Dart roll.

dart-lang/sdk#34872


Original change's description:
> [build] Debug means debug!
> 
> Bug: dart-lang/sdk#29458
> Change-Id: Ic7d4cb81c65da9c7a8970431b4e02f3d2d19e556
> Reviewed-on: https://dart-review.googlesource.com/c/77980
> Reviewed-by: Siva Annamalai <[email protected]>
> Reviewed-by: Zach Anderson <[email protected]>
> Commit-Queue: Ryan Macnak <[email protected]>

[email protected],[email protected],[email protected]

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

Bug: dart-lang/sdk#29458
Change-Id: I45ea9e1408d71e79d28d8ff0f5aea7649d5fb2ac
Reviewed-on: https://dart-review.googlesource.com/c/80920
Reviewed-by: Aart Bik <[email protected]>
Commit-Queue: Aart Bik <[email protected]>

runtime/runtime_args.gni

@@ -6,7 +6,7 @@ declare_args() {
   # Instead of using is_debug, we introduce a different flag for specifying a
   # Debug build of Dart so that clients can still use a Release build of Dart
   # while themselves doing a Debug build.
-  dart_debug = is_debug
+  dart_debug = false
 
   # Set the runtime mode. This affects how the runtime is built and what
   # features it has. Valid values are: