Remove always-on language feature flag: OpenTypeDeclaration

[T-Gro]

LanguageFeature.OpenTypeDeclaration has shipped since F# 5.0 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 OpenTypeDeclaration case in LanguageFeatures.fs and LanguageFeatures.fsi
• Its entry in the features map and its GetFeatureString arm
• The featureOpenTypeDeclaration string in FSComp.txt
• Every SupportsFeature LanguageFeature.OpenTypeDeclaration / checkLanguageFeatureAndRecover … OpenTypeDeclaration guard — keep only the enabled path and delete the legacy branch
• Adjust affected tests and baselines accordingly

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

Version mapping

fsharp/src/Compiler/Facilities/LanguageFeatures.fs

Line 166 in ea778bb

LanguageFeature.OpenTypeDeclaration, languageVersion50

Feature string

fsharp/src/Compiler/FSComp.txt

Line 1572 in ea778bb

featureOpenTypeDeclaration,"open type declaration"

Part of #20139