Remove always-on language feature flag: RelaxWhitespace

[T-Gro]

LanguageFeature.RelaxWhitespace has shipped since F# 4.7 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 RelaxWhitespace case in LanguageFeatures.fs and LanguageFeatures.fsi
• Its entry in the features map and its GetFeatureString arm
• The featureRelaxWhitespace string in FSComp.txt
• Every SupportsFeature LanguageFeature.RelaxWhitespace / checkLanguageFeatureAndRecover … RelaxWhitespace guard — keep only the enabled path and delete the legacy branch
• Adjust affected tests and baselines accordingly

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

Version mapping

fsharp/src/Compiler/Facilities/LanguageFeatures.fs

Line 157 in ea778bb

LanguageFeature.RelaxWhitespace, languageVersion47

Feature string

fsharp/src/Compiler/FSComp.txt

Line 1569 in ea778bb

featureRelaxWhitespace,"whitespace relaxation"

Part of #20139