Revert "[BUILD] - Initial CL to unfork the NNBD Dart SDK"

This reverts commit cab69e7706.

Reason for revert: It was a temporary submit

Original change's description:
> [BUILD] - Initial CL to unfork the NNBD Dart SDK
> NOTE: This is a trial submit and will be reverted immediately.
> 
> - Flips the flag from --nnbd to --no-nnbd so that by default it builds
>   the NNBD version
> - using the --no-nnbd flag results in the SDK being built in a
>   directory which has the 'Legacy' suffix added to it
>   (e.g: out/DebugX64Legacy)
> - the '--enable-experiment=non-nullable' flag still needs to be passed
>   in during execution so that CFE runs in that mode. This is different
>   from the 'null_safety' flag
> 
> Change-Id: I7d25d9710818af5919c0bb83abe51153172f5886
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/144880
> Reviewed-by: Siva Annamalai <asiva@google.com>

TBR=asiva@google.com

Change-Id: Ib0f99fc1a42c9c7a8b21f8542f4a35dba9105d5c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/144900
Reviewed-by: Siva Annamalai <asiva@google.com>
Commit-Queue: Siva Annamalai <asiva@google.com>

sdk_args.gni

@@ -5,11 +5,9 @@
 _dart_root = get_path_info(".", "abspath")
 
 declare_args() {
-  # Whether to build a Legacy SDK using Legacy core libraries.
-  # TODO(38701): Remove dont_use_nnbd once the NNBD SDK is stable/performant
-  # and there is no need to build a legacy version of the SDK for comparison
-  # purposes.
-  dont_use_nnbd = false
+  # Whether to use the NNBD fork of the SDK core libraries.
+  # TODO(#38701): Remove this when the fork has been merged back in.
+  use_nnbd = false
 
   # Absolute path to the .git folder.
   #