Remove Dependabot groups

[FloEdelmann]

Since #519, we don't list @typescript-eslint/eslint-plugin and @typescript-eslint/parser separately in package.json, only the single typescript-eslint dependency (see also https://typescript-eslint.io/blog/announcing-typescript-eslint-v7/#switching-to-typescript-eslint). Thus, grouping in Dependabot is no longer necessary.