[ Observatory ] Remove Observatory from the Dart VM

Also cleans up some references to Observatory in various places.

Work towards https://github.com/dart-lang/sdk/issues/50233

TEST=N/A
CoreLibraryReviewExempt: Not modifying public core libraries.
Change-Id: I1f36b4e6f1fd9a59a579d719aafa599906eedb3f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/429141
Reviewed-by: Siva Annamalai <asiva@google.com>
Commit-Queue: Ben Konyi <bkonyi@google.com>

runtime/include/dart_api.h

@@ -930,6 +930,8 @@ typedef struct {
   /**
    * A function to be called by the service isolate when it requires the
    * vmservice assets archive. See Dart_GetVMServiceAssetsArchive.
+   * 
+   * This field is deprecated and has no effect.
    */
   Dart_GetVMServiceAssetsArchive get_service_assets;