NFC: remove dead code

Author: kavon

include/swift/AST/ASTMangler.h

@@ -72,11 +72,6 @@ class ASTMangler : public Mangler {
   /// If enabled, marker protocols can be encoded in the mangled name.
   bool AllowMarkerProtocols = true;
 
-  /// Whether the mangling predates concurrency, and therefore shouldn't
-  /// include concurrency features such as global actors or @Sendable
-  /// function types.
-  bool Preconcurrency = false;
-
   /// If enabled, declarations annotated with @_originallyDefinedIn are mangled
   /// as if they're part of their original module. Disabled for debug mangling,
   /// because lldb wants to find declarations in the modules they're currently