Disable Android system auto backup (closes #5114)

In practice this feature:
- Just didn't work
- Magically worked (sometimes; see first point)
- Restored something potentially too old and totally messed up the app

app/src/main/res/xml/backup_rules.xml

@@ -1,6 +0,0 @@
-<?xml version="1.0" encoding="utf-8"?>
-<full-backup-content>
-    <include
-        domain="database"
-        path="tachiyomi.db" />
-</full-backup-content>