Remove deprecated options in [tool.darker]

[akaihola]

These options were deprecated in Darker 2.1.1 (see #578). They can still be used as command line arguments or in the [tool.black] section of the configuration file.

• remove the options from DarkerConfig
• resurrect tests but modify them to assert that the options are ignored in [tool.darker] (or cause an error)

See also:

• --no-skip-string-normalization option #50
• Add support for black's --skip-magic-trailing-comma flag #152
• pyproject.toml options with hyphens are ignored #502

🟠 Pending review is delaying Darker 3.0.0 release.

[akaihola]

@CorreyL contributed --skip-string-normalization in #3.

@hauntsaninja originally requested the --skip-magic-trailing-comma option in #152, and @talhajunaidd implemented it in #153.

Both of those are still there as a command line option, but not supported in pyproject.toml's [tool.darker] section any more. Put them in [tool.black] instead.

@dwt noted in #502 that hyphenated spellings of configuration options like skip-magic-trailing-comma = and skip-string-normalization = used to be ignored by Darker in pyproject.toml. Those two are not a concern of Darker's any more.

Would any of you guys be willing to take another look at this PR to complete its review?

[dwt]

Looks good, but I would like the information about where the options move to to be include din the deprecation warnings to spare the user a research step and give him the solution immediately.