Remove generated launcher icons as PNG files

The seem to be generated during build now and appear in
build/generated/res/pngs/debug/drawable-*

mailbox/src/main/res/drawable/ic_launcher_background.xml

-<?xml version="1.0" encoding="utf-8"?>
-<vector
-    android:height="108dp"
-    android:width="108dp"
-    android:viewportHeight="108"
-    android:viewportWidth="108"
-    xmlns:android="http://schemas.android.com/apk/res/android">
-    <path android:fillColor="#ffffff"
-          android:pathData="M0,0h108v108h-108z"/>
-</vector>