Remove iOS stubs (#3490)

Plugins that don't actually support iOS are no longer required to have
an iOS stub to prevent build failures. This removes all iOS stubs from
plugins that don't support iOS.

.cirrus.yml

@@ -201,7 +201,7 @@ task:
         - flutter channel $CHANNEL
         - flutter upgrade
         - ./script/incremental_build.sh build-examples --ipa
-        - ./script/incremental_build.sh drive-examples
+        - ./script/incremental_build.sh drive-examples --ios
         - ./script/incremental_build.sh xctest --target RunnerUITests --skip $PLUGINS_TO_SKIP_XCTESTS --ios-destination "platform=iOS Simulator,name=iPhone 11,OS=14.3"
 
 task: