Remove always-on language feature flag: StaticLetInRecordsDusEmptyTypes

[T-Gro]

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

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

Version mapping

fsharp/src/Compiler/Facilities/LanguageFeatures.fs

Line 217 in ea778bb

LanguageFeature.StaticLetInRecordsDusEmptyTypes, languageVersion80

Feature string

fsharp/src/Compiler/FSComp.txt

Line 1744 in ea778bb

featureStaticLetInRecordsDusEmptyTypes,"Allow static let bindings in union, record, struct, non-incremental-class types"

Part of #20139