Update .backportrc.json: remove 7.x preset

[joepeeples]

Removes syntax that creates a pre-selected (or "checked")7.x option every time you run the backport tool. We used to do this during the 7.x series, but we no longer maintain such a branch and don't need it as a pre-selected option.

Backport tool docs: https://github.com/sorenlouv/backport/blob/main/docs/config-file-options.md#targetbranchchoices-required
(The documented setting names are slightly different from what's in our config file. Maybe they were renamed at some point but maintain backwards compatibility with our names?)

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[joepeeples]

FYI: I don't think we need to backport this change to previous branches, since we usually run the backport tool from main, not a versioned branch.