iOS: Fixes #2301: Removed filesystem sync option, which was not supported

ReactNativeClient/lib/BaseSyncTarget.js

@@ -33,6 +33,11 @@ class BaseSyncTarget {
 		return this.db_;
 	}
 
+	// If [] is returned it means all platforms are supported
+	static unsupportedPlatforms() {
+		return [];
+	}
+
 	async isAuthenticated() {
 		return false;
 	}