Remove always-on language feature flag: FixedIndexSlice3d4d

[T-Gro]

LanguageFeature.FixedIndexSlice3d4d has shipped since F# 5.0 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 FixedIndexSlice3d4d case in LanguageFeatures.fs and LanguageFeatures.fsi
• Its entry in the features map and its GetFeatureString arm
• The featureFixedIndexSlice3d4d string in FSComp.txt
• Every SupportsFeature LanguageFeature.FixedIndexSlice3d4d / checkLanguageFeatureAndRecover … FixedIndexSlice3d4d guard — keep only the enabled path and delete the legacy branch
• Adjust affected tests and baselines accordingly

Consequence: --disableLanguageFeature:FixedIndexSlice3d4d is no longer a recognised feature name.

Version mapping

fsharp/src/Compiler/Facilities/LanguageFeatures.fs

Line 161 in ea778bb

LanguageFeature.FixedIndexSlice3d4d, languageVersion50

Feature string

fsharp/src/Compiler/FSComp.txt

Line 1576 in ea778bb

featureFixedIndexSlice3d4d,"fixed-index slice 3d/4d"

Part of #20139