flatpak: Delete CMake-specific build files

Fallout from 70bc4806

!105
3 jobs for wip/rishi/delete-cmake-build-files in 15 minutes and 21 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #314294
flatpak

00:13:54

 
  Review
passed #314295
review

00:00:29

passed #314296
allowed to fail manual
stop_review

00:00:57