[Decl] remove not used and bad-idea isIsolated function

Author: ktoso

include/swift/AST/Decl.h

@@ -6414,9 +6414,6 @@ class AbstractFunctionDecl : public GenericContext, public ValueDecl {
   /// A function is concurrent if it has the @Sendable attribute.
   bool isSendable() const;
 
-  /// Determine if function has 'nonisolated' attribute
-  bool isNonisolated() const;
-
   /// Returns true if the function is a suitable 'async' context.
   ///
   /// Functions that are an 'async' context can make calls to 'async' functions.

lib/AST/Decl.cpp

@@ -7781,10 +7781,6 @@ bool AbstractFunctionDecl::isSendable() const {
   return getAttrs().hasAttribute<SendableAttr>();
 }
 
-bool AbstractFunctionDecl::isNonisolated() const {
-  return getAttrs().hasAttribute<NonisolatedAttr>();
-}
-
 bool AbstractFunctionDecl::isBackDeployed() const {
   if (getAttrs().hasAttribute<BackDeployAttr>())
     return true;