Remove always-on language feature flag: StructActivePattern

[T-Gro]

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

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

Version mapping

fsharp/src/Compiler/Facilities/LanguageFeatures.fs

Line 179 in ea778bb

LanguageFeature.StructActivePattern, languageVersion60

Feature string

fsharp/src/Compiler/FSComp.txt

Line 1585 in ea778bb

featureStructActivePattern,"struct representation for active patterns"

Part of #20139