Don't handle back key twice

Volker Krauserequested to merge
work/vkrause/fix-double-page-pop into master

The back key is also handled by a shortcut handler a few lines down, together with the forward key.

This fixes a double page pop when pressing the back key (or using the back gesture) on Android.

This went unnoticed as one of those didn't actually work, but Qt 6.11 apparently fixed this, breaking applications with a page stack depth > 1.

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