[Frontend] NFC: Remove experimental flag for runtime discoverable attributes feature

Author: xedin

include/swift/Basic/Features.def

@@ -196,10 +196,6 @@ EXPERIMENTAL_FEATURE(ASTGenTypes, false)
 /// corresponding syntax tree.
 EXPERIMENTAL_FEATURE(BuiltinMacros, false)
 
-/// Whether to enable experimental @runtimeMetadata feature which allows to
-/// declare an attribute which is discoverable and constructable at runtime.
-EXPERIMENTAL_FEATURE(RuntimeDiscoverableAttrs, false)
-
 /// Import C++ class templates as semantically-meaningless symbolic
 /// Swift types and C++ methods as symbolic functions with blank
 /// signatures.