dix: Remove WindowRec::backStorage

This is only being set, never read.
3 jobs for simplify-backstorage in 9 minutes and 11 seconds (queued for 1 minute and 26 seconds)
Status Job ID Name Coverage
  Docker Image
passed #185979
debian-testing

00:00:37

 
  Build And Test
passed #185980
autotools-build-and-test

00:08:28

passed #185981
meson-build-and-test

00:01:31