Remove always-on language feature flag: AccessorFunctionShorthand

[T-Gro]

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

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

Version mapping

fsharp/src/Compiler/Facilities/LanguageFeatures.fs

Line 199 in ea778bb

LanguageFeature.AccessorFunctionShorthand, languageVersion80

Feature string

fsharp/src/Compiler/FSComp.txt

Line 1750 in ea778bb

featureAccessorFunctionShorthand,"underscore dot shorthand for accessor only function"

Part of #20139