[vm/concurrency] Remove Dart_IsolateFlags.entry_points / Dart_QualifiedFunctionName from Dart C API

This CL removes the unused entry points in the C API. This is a breaking
API change but we don't promise normal Dart C API stability and existing
known embedders don't specify this flag.

TEST=Pure refactoring - relying on existing test coverage.

Issue https://github.com/dart-lang/sdk/issues/36097

Change-Id: Ic27662743e48f5f5cfbf770baf2cd255ed3f15b5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/177121
Reviewed-by: Alexander Aprelev <aam@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>

runtime/include/dart_api.h

@@ -588,12 +588,6 @@ DART_EXPORT void Dart_UpdateFinalizableExternalSize(
  */
 DART_EXPORT const char* Dart_VersionString();
 
-typedef struct {
-  const char* library_uri;
-  const char* class_name;
-  const char* function_name;
-} Dart_QualifiedFunctionName;
-
 /**
  * Isolate specific flags are set when creating a new isolate using the
  * Dart_IsolateFlags structure.
@@ -610,7 +604,6 @@ typedef struct {
   bool use_field_guards;
   bool use_osr;
   bool obfuscate;
-  Dart_QualifiedFunctionName* entry_points;
   bool load_vmservice_library;
   bool copy_parent_code;
   bool null_safety;