Clean up .gitignore files (flutter#153060)

This removes some unnecessary `.gitignore` files that are already covered by the top-level `.gitignore` file.

This ensures that `flutter run`ning examples in this repo doesn't result in modifications by the new Swift Package Manager migration.

Follow up to flutter#152766

Part of flutter#148018

Author: loic-sharma

.gitignore

@@ -113,11 +113,13 @@ unlinked_spec.ds
 **/xcuserdata/
 
 # Windows
+**/windows/flutter/ephemeral/
 **/windows/flutter/generated_plugin_registrant.cc
 **/windows/flutter/generated_plugin_registrant.h
 **/windows/flutter/generated_plugins.cmake
 
 # Linux
+**/linux/flutter/ephemeral/
 **/linux/flutter/generated_plugin_registrant.cc
 **/linux/flutter/generated_plugin_registrant.h
 **/linux/flutter/generated_plugins.cmake