Remove always-on language feature flag: ArithmeticInLiterals

[T-Gro]

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

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

Version mapping

fsharp/src/Compiler/Facilities/LanguageFeatures.fs

Line 205 in ea778bb

LanguageFeature.ArithmeticInLiterals, languageVersion80

Feature string

fsharp/src/Compiler/FSComp.txt

Line 1598 in ea778bb

featureArithmeticInLiterals,"Arithmetic and logical operations in literals, enum definitions and attributes"

Part of #20139