remove deprecated no-op targetSandboxVersion

The downgrade check was removed in Android 9 and the feature was
turned into a deprecated no-op in Android 10.

Author: thestinger

app/src/main/AndroidManifest.xml

@@ -1,6 +1,5 @@
 <?xml version="1.0" encoding="utf-8"?>
-<manifest xmlns:android="http://schemas.android.com/apk/res/android"
-        android:targetSandboxVersion="2">
+<manifest xmlns:android="http://schemas.android.com/apk/res/android">
     <original-package android:name="org.grapheneos.pdfviewer" />
 
     <application android:name=".App"