Remove always-on language feature flag: MatchNotAllowedForUnionCaseWithNoData

[T-Gro]

LanguageFeature.MatchNotAllowedForUnionCaseWithNoData has shipped since F# 8.0 and is enabled for every selectable language version (the minimum accepted --langversion is 8.0). Its feature flag can therefore never be turned off, so it is dead configuration. This issue removes the flag and collapses the guarded code to its enabled behaviour.

What to remove

• The MatchNotAllowedForUnionCaseWithNoData case in LanguageFeatures.fs and LanguageFeatures.fsi
• Its entry in the features map and its GetFeatureString arm
• The featureMatchNotAllowedForUnionCaseWithNoData string in FSComp.txt
• Every SupportsFeature LanguageFeature.MatchNotAllowedForUnionCaseWithNoData / checkLanguageFeatureAndRecover … MatchNotAllowedForUnionCaseWithNoData guard — keep only the enabled path and delete the legacy branch
• Adjust affected tests and baselines accordingly

Consequence: --disableLanguageFeature:MatchNotAllowedForUnionCaseWithNoData is no longer a recognised feature name.

Version mapping

fsharp/src/Compiler/Facilities/LanguageFeatures.fs

Line 200 in ea778bb

LanguageFeature.MatchNotAllowedForUnionCaseWithNoData, languageVersion80

Feature string

fsharp/src/Compiler/FSComp.txt

Line 1594 in ea778bb

featureMatchNotAllowedForUnionCaseWithNoData,"Pattern match discard is not allowed for union case that takes no data."

Part of #20139