Remove package:macros and package:_macros from pubspec.yaml (flutter#166749)

Update Flutter for the upcoming removal of pkg/_macros and pkg/macros
from Dart SDK: https://dart-review.googlesource.com/c/sdk/+/420680.

Author: alexmarkov

engine/src/flutter/pubspec.yaml

@@ -166,8 +166,6 @@ dependency_overrides:
     path: ./third_party/pkg/googleapis/discoveryapis_commons
   _fe_analyzer_shared:
     path: ./third_party/dart/pkg/_fe_analyzer_shared
-  _macros:
-    path: ./third_party/dart/pkg/_macros
   analyzer:
     path: ./third_party/dart/pkg/analyzer
   archive:
@@ -222,8 +220,6 @@ dependency_overrides:
     path: ./third_party/dart/pkg/kernel
   logging:
     path: ./third_party/dart/third_party/pkg/core/pkgs/logging
-  macros:
-    path: ./third_party/dart/pkg/macros
   matcher:
     path: ./third_party/dart/third_party/pkg/test/pkgs/matcher
   meta:

engine/src/flutter/web_sdk/pubspec.yaml

@@ -18,8 +18,6 @@ dev_dependencies:
 dependency_overrides: # Must include all transitive dependencies from the "any" packages above.
   _fe_analyzer_shared:
     path: ../third_party/dart/pkg/_fe_analyzer_shared
-  _macros:
-    path: ../third_party/dart/pkg/_macros
   analyzer:
     path: ../third_party/dart/pkg/analyzer
   args:
@@ -38,8 +36,6 @@ dependency_overrides: # Must include all transitive dependencies from the "any"
     path: ../third_party/dart/third_party/pkg/tools/pkgs/file
   glob:
     path: ../third_party/dart/third_party/pkg/tools/pkgs/glob
-  macros:
-    path: ../third_party/dart/pkg/macros
   meta:
     path: ../third_party/dart/pkg/meta
   package_config: