Avoid duplicating the default values of e.g. GitHubDefectsConfiguration.labelFilter

[sschuberth]

          This now duplicates the default value of `GitHubDefectsConfiguration.labelFilter`, which IMO is not so nice. In [my old draft](https://github.com/oss-review-toolkit/ort/pull/6613) I tried to [solve this by extracting default values to constants](https://github.com/oss-review-toolkit/ort/pull/6613/commits/0db7e0f29d750b6e336cff02658f1673b1e8673d#diff-c683de376a76556ce1a5c1d790017956b5726c1b270ca08a1e3e2d61a76054cdR106-R112) (not fully implemented, also see the last commit in that PR).

This raises the general question where default values should be applied. I see basically tree options:

1. in parseConfig(),
2. in the constructor of the output config class,
3. in the consumer of the config class.

My current preference would be 1., and I'd propose a clean-up commit that removes the default parameters from GitHubDefectsConfiguration's constructor.

Originally posted by @sschuberth in #7690 (comment)

[sschuberth]

Obsolete as of #8994.