Remove always-on language feature flag: WarningWhenTailRecAttributeButNonTailRecUsage

[T-Gro]

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

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

Version mapping

fsharp/src/Compiler/Facilities/LanguageFeatures.fs

Line 216 in ea778bb

LanguageFeature.WarningWhenTailRecAttributeButNonTailRecUsage, languageVersion80

Feature string

fsharp/src/Compiler/FSComp.txt

Line 1609 in ea778bb

featureChkNotTailRecursive,"Raises warnings if a member or function has the 'TailCall' attribute, but is not being used in a tail recursive way."

Part of #20139