No longer include .packages in created .gitignore files (#133484)

The `.packages` file was deprecated in Dart 2.8 and slowly discontinued until support being fully removed in Dart 2.19. The file will no longer be created, so it can be safely dropped from the generated `.gitignore` files.

packages/flutter_tools/templates/app_shared/.gitignore.tmpl

@@ -27,7 +27,6 @@ migrate_working_dir/
 .dart_tool/
 .flutter-plugins
 .flutter-plugins-dependencies
-.packages
 .pub-cache/
 .pub/
 /build/

packages/flutter_tools/templates/module/common/.gitignore.tmpl

@@ -1,7 +1,6 @@
 .DS_Store
 .dart_tool/
 
-.packages
 .pub/
 
 .idea/

packages/flutter_tools/templates/package/.gitignore.tmpl

@@ -26,5 +26,4 @@ migrate_working_dir/
 /pubspec.lock
 **/doc/api/
 .dart_tool/
-.packages
 build/

packages/flutter_tools/templates/plugin_shared/.gitignore.tmpl

@@ -26,5 +26,4 @@ migrate_working_dir/
 /pubspec.lock
 **/doc/api/
 .dart_tool/
-.packages
 build/