Remove always-on language feature flag: RelaxWhitespace2

[T-Gro]

LanguageFeature.RelaxWhitespace2 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 RelaxWhitespace2 case in LanguageFeatures.fs and LanguageFeatures.fsi
• Its entry in the features map and its GetFeatureString arm
• The featureRelaxWhitespace2 string in FSComp.txt
• Every SupportsFeature LanguageFeature.RelaxWhitespace2 / checkLanguageFeatureAndRecover … RelaxWhitespace2 guard — keep only the enabled path and delete the legacy branch
• Adjust affected tests and baselines accordingly

Consequence: --disableLanguageFeature:RelaxWhitespace2 is no longer a recognised feature name.

Version mapping

fsharp/src/Compiler/Facilities/LanguageFeatures.fs

Line 175 in ea778bb

LanguageFeature.RelaxWhitespace2, languageVersion60

Feature string

fsharp/src/Compiler/FSComp.txt

Line 1586 in ea778bb

featureRelaxWhitespace2,"whitespace relaxation v2"

Part of #20139