Remove deprecated test-utils-core package

Signed-off-by: Adam Harvey <adaharve@cisco.com>

docs/getting-started/project-structure.md

@@ -166,13 +166,9 @@ are separated out into their own folder, see further down.
   plugin and [techdocs-cli](https://github.com/backstage/techdocs-cli).
 
 - [`test-utils/`](https://github.com/backstage/backstage/tree/master/packages/test-utils) -
-  This package contains more general purpose testing facilities for testing a
+  This package contains general purpose testing facilities for testing a
   Backstage App or its plugins.
 
-- [`test-utils-core/`](https://github.com/backstage/backstage/tree/master/packages/test-utils-core) -
-  This package contains specific testing facilities used when testing Backstage
-  core internals.
-
 - [`theme/`](https://github.com/backstage/backstage/tree/master/packages/theme) -
   Holds the Backstage Theme.
 

docs/overview/stability-index.md

@@ -175,15 +175,6 @@ Utilities for writing tests for Backstage plugins and apps.
 
 Stability: `2`
 
-### `test-utils-core` [GitHub](https://github.com/backstage/backstage/tree/master/packages/test-utils-core/)
-
-Internal testing utilities that are separated out for usage in
-@backstage/core-app-api and @backstage/core-plugin-api. All exports are
-re-exported by @backstage/test-utils. This package should not be depended on
-directly.
-
-Stability: See @backstage/test-utils
-
 ### `theme` [GitHub](https://github.com/backstage/backstage/tree/master/packages/theme/)
 
 The core Backstage MUI theme along with customization utilities.