Remove old fields from analysis_options.yaml

[srawlins]

I see support in analyzer's AnalysisOptions class, support for:

• enableSuperMixins
• enablePreviewDart2
• optional-checks (maybe just the Chrome OS Manifest checks?)
• strong-mode (declaration-casts, implicit-casts, implicit-dynamic), covered by Remove analyzer's deprecated analysis modes: no-implicit-casts and no-implicit-dynamic #50679
• propagate-linter-exceptions

We could remove at least enableSuperMixins, enablePreviewDart2, and strong-mode. Not sure about optional-checks or propagate-linter-exceptions.

[srawlins]

We have already deprecated enableSuperMixins (with AnalysisOptionsHintCode.SUPER_MIXINS_SETTING_DEPRECATED) and enablePreviewDart2 (with AnalysisOptionsHintCode.PREVIEW_DART_2_SETTING_DEPRECATED).

[srawlins]

cc @pq do you know what propagate-linter-exceptions is?

[pq]

Yeah. This is something @stereotype441 added to improve internal builds and testing (more context: https://dart-review.googlesource.com/c/sdk/+/208141).

[srawlins]

Ok cool, we'll keep it!