dix: Remove WindowRec::backStorage

This is only being set, never read.
3 jobs for simplify-backstorage in 7 minutes and 21 seconds (queued for 1 second)
Status Job ID Name Coverage
  Docker Image
passed #234606
debian-testing

00:01:03

 
  Build And Test
passed #234607
autotools-build-and-test

00:06:18

passed #234608
meson-build-and-test

00:01:48