gtkentryaccessible: Fix text-changed::delete length

The second parameter of the text-changed::delete event is to be the length,
not the end position.  This fixes spurious text removals in brltty
screen reading.

(cherry picked from commit 209f908a)
4 jobs for text-changed-delete-4 in 37 minutes and 41 seconds (queued for 1 second)
Status Name Job ID Coverage
  Build
passed gtk #21300

00:10:06

 
  Flatpak
passed flatpak:demo #21301

00:04:54

passed flatpak:widget-factory #21313

00:22:40

failed flatpak:widget-factory #21302

00:01:42