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ARROW-7666: [Packaging][deb] Always use Ninja to reduce build time

debhelper 11.2 or later supports `cmake+ninja`: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=895044
We require debhelper 12 or later in `debian/control`.
So we can always use `cmake+ninja`.

Closes #6272 from kou/packaging-debian-use-ninja and squashes the following commits:

974b9191b <Sutou Kouhei>  Always use Ninja to reduce build time

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>