Apply multi-arch hints

Apply multi-arch hints.

• liblightdm-gobject-dev: Drop Multi-Arch: same.This fixes: liblightdm-gobject-dev conflicts on 7 files starting with /usr/share/gtk-doc/html/lightdm-gobject-1/ on any two of amd64, arm64, i386, mips64el, and 3 more. (file-conflict)
• gir1.2-lightdm-1: Drop Multi-Arch: same.This fixes: gir1.2-lightdm-1 conflicts on /usr/lib/girepository-1.0/LightDM-1.typelib on amd64, arm64, and 5 more with 28 combinations. (file-conflict)

This merge proposal was created automatically by the Janitor bot.

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Build and test logs for this branch can be found at https://janitor.debian.net/multiarch-fixes/pkg/lightdm/0a5dda56-5676-47e1-b243-b49d0aefab2e.

These changes affect the binary packages; see the debdiff

You can also view the diffoscope diff (unfiltered).