[ VM ] Remove dead fork of Observatory from runtime/

Observatory was moved to pkg/observatory in a previous CL.

TEST=N/A

Change-Id: Ie0e7cb71799408cefe30d2c72b4dfd7bd43747ce
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/450221
Auto-Submit: Ben Konyi <bkonyi@google.com>
Commit-Queue: Ben Konyi <bkonyi@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>

pubspec.yaml

@@ -317,4 +317,4 @@ dependency_overrides:
   # observatory needs a migration to language version 3.6 before it can be
   # included in the workspace. (If it is not deleted before)
   observatory:
-    path: runtime/observatory
+    path: pkg/observatory