Remove pkg/diagnostic - this is no longer used.

Change-Id: I755f47c30ef015da53ace4bd0464b3f14c5219d2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/233448
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Devon Carew <devoncarew@google.com>

.dart_tool/package_config.json

@@ -11,7 +11,7 @@
     "constraint, update this by running tools/generate_package_config.dart."
   ],
   "configVersion": 2,
-  "generated": "2022-02-16T14:11:48.394602",
+  "generated": "2022-02-16T17:23:35.575981",
   "generator": "tools/generate_package_config.dart",
   "packages": [
     {
@@ -288,12 +288,6 @@
       "packageUri": "lib/",
       "languageVersion": "2.12"
     },
-    {
-      "name": "diagnostic",
-      "rootUri": "../pkg/diagnostic",
-      "packageUri": "lib/",
-      "languageVersion": "2.0"
-    },
     {
       "name": "expect",
       "rootUri": "../pkg/expect",