rm manifest place holders

This could have been removed in the fluter 3.0.0 as the
OneSignal-Android-SDK 4.0.0 no loner uses these.

Author: jkasten2

android/build.gradle

@@ -36,41 +36,3 @@ android {
 dependencies {
     api 'com.onesignal:OneSignal:4.6.3'
 }
-
-// Adds required manifestPlaceholders keys to allow mainifest merge gradle step to complete
-// The OneSignal app id should be set in your JS code.
-// Google project number / FCM Sender ID will be pulled in from the OneSignal dashbaord
-class DefaultManifestPlaceHolders {
-    static final MANIFEST_PLACEHOLDERS_DEFAULTS = [
-        onesignal_app_id: '${onesignal_app_id}',
-        onesignal_google_project_number: '${onesignal_google_project_number}'
-    ]
-
-    static void addManifestToAppProject(Project proj) {
-        def androidApp = proj.android
-        MANIFEST_PLACEHOLDERS_DEFAULTS.each { defKey, defValue ->
-            if (!androidApp.defaultConfig.manifestPlaceholders.containsKey(defKey)) {
-                androidApp.defaultConfig.manifestPlaceholders[defKey] = defValue
-
-                androidApp.buildTypes.each { buildType ->
-                    if (!buildType.manifestPlaceholders.containsKey(defKey))
-                        buildType.manifestPlaceholders[defKey] = defValue
-                }
-            }
-        }
-    }
-}
-
-rootProject.childProjects.each { projName, proj ->
-    if (projName != 'app' && projName != 'onesignal_flutter')
-        return
-
-    if (proj.hasProperty('android')) {
-        DefaultManifestPlaceHolders.addManifestToAppProject(proj)
-        return
-    }
-
-    proj.afterEvaluate {
-        DefaultManifestPlaceHolders.addManifestToAppProject(proj)
-    }
-}