Remove the codecov badge, we're not using codecov anymore

Refs #1542

Author: matthiask

README.rst

@@ -16,10 +16,6 @@ Django Debug Toolbar |latest-version|
    :target: https://github.com/jazzband/django-debug-toolbar/actions
    :alt: Build Status
 
-.. |codecov| image:: https://codecov.io/gh/jazzband/django-debug-toolbar/branch/main/graph/badge.svg
-   :target: https://codecov.io/gh/jazzband/django-debug-toolbar
-   :alt: Test coverage status
-
 .. |docs| image:: https://img.shields.io/readthedocs/django-debug-toolbar/latest.svg
    :target: https://readthedocs.org/projects/django-debug-toolbar/
    :alt: Documentation status