system: Remove uses of the 'title' field of <file-system>.
* gnu/system/install.scm (installation-os): Remove uses of the 'title'
field of 'file-system'; use 'file-system-label' as appropriate.
* gnu/system/vm.scm (system-disk-image, system-qemu-image): Likewise.
* gnu/tests.scm (%simple-os): Likewise.
* gnu/tests/install.scm (%minimal-os, %minimal-extlinux-os)
(%minimal-os-on-vda, %separate-home-os, %separate-store-os)
(%raid-root-os, %encrypted-root-os, %btrfs-root-os): Likewise.
* gnu/build/shepherd.scm (default-mounts)[tmpfs]: Likewise.
* tests/guix-system.sh: Likewise.
* tests/system.scm (%root-fs): Likewise.
("operating-system-boot-mapped-devices, implicit dependency"): Likewise.
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