[CFE] Don't call enableNative when creating BodyBuilderImpl - we never use the field anyway

When compiling a big internal app this reduces the wall-clock time from
`2m21.756s` to `1m43.671s`.

Change-Id: I0e89c0c41fe59d2fb3541a50c8c7e2963b0bff47
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/501461
Commit-Queue: Jens Johansen <jensj@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>

pkg/front_end/lib/src/kernel/body_builder.dart

@@ -189,8 +189,6 @@ class BodyBuilderImpl extends StackListenerImpl
 
   final LocalScope enclosingScope;
 
-  final bool enableNative;
-
   // TODO(ahe): Consider renaming [uri] to 'partUri'.
   @override
   final Uri uri;
@@ -372,9 +370,6 @@ class BodyBuilderImpl extends StackListenerImpl
     required this.extensionScope,
     required ThisVariable? internalThisVariable,
   }) : _context = context,
-       enableNative = libraryBuilder.loader.target.backendTarget.enableNative(
-         libraryBuilder.importUri,
-       ),
        benchmarker = libraryBuilder.loader.target.benchmarker,
        _localScopes = new LocalStack([enclosingScope]),
        _labelScopes = new LocalStack([new LabelScopeImpl()]),