Remove always-on language feature flag: WarningWhenMultipleRecdTypeChoice

[T-Gro]

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

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

Version mapping

fsharp/src/Compiler/Facilities/LanguageFeatures.fs

Line 213 in ea778bb

LanguageFeature.WarningWhenMultipleRecdTypeChoice, languageVersion80

Feature string

fsharp/src/Compiler/FSComp.txt

Line 1606 in ea778bb

featureWarningWhenMultipleRecdTypeChoice,"Raises warnings when multiple record type matches were found during name resolution because of overlapping field names."

Part of #20139