Remove always-on language feature flag: ImplicitYield

[T-Gro]

LanguageFeature.ImplicitYield has shipped since F# 4.7 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 ImplicitYield case in LanguageFeatures.fs and LanguageFeatures.fsi
• Its entry in the features map and its GetFeatureString arm
• The featureImplicitYield string in FSComp.txt
• Every SupportsFeature LanguageFeature.ImplicitYield / checkLanguageFeatureAndRecover … ImplicitYield guard — keep only the enabled path and delete the legacy branch
• Adjust affected tests and baselines accordingly

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

Version mapping

fsharp/src/Compiler/Facilities/LanguageFeatures.fs

Line 158 in ea778bb

LanguageFeature.ImplicitYield, languageVersion47

Feature string

fsharp/src/Compiler/FSComp.txt

Line 1571 in ea778bb

featureImplicitYield,"implicit yield"

Part of #20139