Remove always-on language feature flag: AdditionalTypeDirectedConversions

[T-Gro]

LanguageFeature.AdditionalTypeDirectedConversions has shipped since F# 6.0 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 AdditionalTypeDirectedConversions case in LanguageFeatures.fs and LanguageFeatures.fsi
• Its entry in the features map and its GetFeatureString arm
• The featureAdditionalImplicitConversions string in FSComp.txt
• Every SupportsFeature LanguageFeature.AdditionalTypeDirectedConversions / checkLanguageFeatureAndRecover … AdditionalTypeDirectedConversions guard — keep only the enabled path and delete the legacy branch
• Adjust affected tests and baselines accordingly

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

Version mapping

fsharp/src/Compiler/Facilities/LanguageFeatures.fs

Line 174 in ea778bb

LanguageFeature.AdditionalTypeDirectedConversions, languageVersion60

Feature string

fsharp/src/Compiler/FSComp.txt

Line 1584 in ea778bb

featureAdditionalImplicitConversions,"additional type-directed conversions"

Part of #20139