Remove always-on language feature flag: NonVariablePatternsToRightOfAsPatterns

[T-Gro]

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

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

Version mapping

fsharp/src/Compiler/Facilities/LanguageFeatures.fs

Line 184 in ea778bb

LanguageFeature.NonVariablePatternsToRightOfAsPatterns, languageVersion60

Feature string

fsharp/src/Compiler/FSComp.txt

Line 1253 in ea778bb

featureNonVariablePatternsToRightOfAsPatterns,"non-variable patterns to the right of 'as' patterns"

Part of #20139