Remove always-on language feature flag: ResumableStateMachines

[T-Gro]

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

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

Version mapping

fsharp/src/Compiler/Facilities/LanguageFeatures.fs

Line 178 in ea778bb

LanguageFeature.ResumableStateMachines, languageVersion60

Feature string

fsharp/src/Compiler/FSComp.txt

Line 1579 in ea778bb

featureResumableStateMachines,"resumable state machines"

Part of #20139