Remove README from the test plugin

The README file was added only to test Core Metadata editing, and it's
doubly unnecessary -- because we use a separate metadata file now,
and because we use a separate test package.

Author: mgorny

tests/artifacts/test-plugin-package/README

@@ -7,4 +7,3 @@ name = "test-plugin-package"
 version = "0"
 description = "Installable plugin for testing"
 requires-python = ">=3.9"
-readme = "README"